So in a supplement book by Genuine fantasy press there is a metaphysical currency called gilt which is equivalent to the hp a creature has. Ex: 76 Max Hp = 76 gilt earned from killing the creature - but!

In my attempt to find a way to trade gilt for coin as a sort of soul currency as a DM I ran across once more the soul cooks in Descent into Avernus and saw that they have the rough gp value of perhaps 500-5000 depending on the level of devil you are bartering with. My question is, what determines the exact value of the soul coin? I know a champion of moradin has a greater soul value than that of a lowest goblin npc but - how do you measure it? If you were to what would you base it off and if the system worked akin to gilt- what would the value of gilt to gold ratio be ? Much like how would you determine the exact value of a soul in a soul coin? 

any ideas and thoughts would be much appreciated as I have a player very interested in these sort of devil dealings and concepts for his Teifling Paladin who worships Kelemvor 

Not all souls get minted as coins. The majority get turned into lemures. I'd imagine that Mammon arranges it so that only souls of an approximate value get minted, with exceptions for high ranking devils who commission special coins.

I mean, the whole purpose of minting coins is to get units of equal value, so the gilt (whatever that is) comparison is inapt.  The only way given to express an exact value is that each and every fully charged soul coin is equal to 72 hours of fuel for a midsize war machine.

What the market price may be will depend on how desperate the seller is and how desperate the buyer is.  If Yeenoghu and 666,000 undead gnolls are coming over the hill and you're the only one with a soul coin to sell, it could be worth everything a given devil had to offer.  Millions.  OTOH, in the security of the Wandering Emporium they're worth 100gp in coins or gems.  If your game pays attention to encumbrance, one dick move would be to for a devil to pay them something like 50K for a handful and then laugh at them trying to drag a half ton of gold through the wasteland.

I've been thinking of doing more with soul coins beyond Avernus myself.  I don't see where you're seeing Soul Coins having any set exchange rate to more traditional D&D currency (gold pieces), nor any intrinsic valuation (i.e. a "hero's soul is worth x time that of a common soul").  

Soul coins are pretty standardized in terms of magical function.  Remember as Thucydides points out, souls that wind up in Hell, regardless of who they were start out as Lemures.  Apparently a portion are set by some sort of Infernal Reserve System (IRS, heh) to be minted into soul coins (to either be obliterated in the fueling of infernal machines, or eventually return to the Styx and become a Lemure if the coin was used differently than infernal combustion engines).  The soul coin can provide a limited number of temp hit points or can answer questions from the perspective of the soul for which the coin was minted to contain.  Devil's traffic in souls all the time, this is another way to turn that traffic into a currency,. They don't seem to have any trading currency outside of Hell, and it's implied within Avernus agents of good planar entities find them an atrocity (they also take a toll on non evil characters who carry more than their CON can handle).  That said, my tentative expansion on the idea posits that soul coins expose a truth that there is a soul economy within the outer planes, which could be destabilized with increased human intervention in the outer planes.  Kelemvor may find such transgression uncool.  Enter the paladin.

The current UA Phantom subclass for Rogues has a similar object called a soul trinket that functions in a similar way.  You might want to read that too for comparison or further your imagination on this.  

I’m unsure I’d agree with that all souls have the same value. The writing in the book specifically says notes on page 127 that  Fhet'Ahla the amnizu offers currency exchange and messengers in exchange for soul coins, preferably those that contained the souls of celestial beings. I’d say that’s an argument for the value of soul coins actually being different and just there not being an exact system made by Wizards who’d rather leave artistic interpretation to individual DMs. I believe this is from the traveling Bazaar of Avernus which has a specific character who will buy soil coins for 100 a piece. Additionally in “Diabolical Deals” appendix a it states different levels of devils buying and selling them for different ammounts

Do you mean this?

Notary Pricing. Fhet’Ahla charges a single soul coin for notary services and collects a single soul coin for exchanges that don’t involve the soul of a celestial. If a celestial is involved, he collects ten soul coins.

That's his fee for acting as notary for the signing of an infernal contract.  The contract has a higher intrinsic value because a celestial is involved, which is reflected in the fee he charges.  The actual units of exchange (the soul coins) are of equal value.

And this part...

A devil might be persuaded to void a contract in exchange for one of the following:

• One or more soul coins (lesser devils will often settle for three, greater devils for nine, and archdevils have no interest in such a paltry alternative)

The devil in this position is already in possession of something you want (presumably rights to your soul).  You want the rights back.  It's a seller's market.  The devil can charge whatever they want and you'll find a way to pay it.  A lesser devil, who might be unimaginative, or might have liquidity problems of his own, sells your soul back to you for three soul coins.  A greater devil has all the time in the world and knows that you'll do anything to get your soul back (you might even do something so corrupt that your soul will end up forfeit anyway), so he charges more.  But each of the soul coins they're willing to accept to void the contract they hold are worth an equal amount.  And if you were to end up with leverage over your counterparty, somehow, you could even end up paying a lot less.

I take your point that souls have different values.  I have to imagine that the ones that are worth more than or less than 72 hours of fuel just don't get turned into coins. 

That makes more sense and I’d have to agree with it now that I understand from your reasoning as well as a clarification of what is written in the book (don’t currently own it). 

So with the system of gilt there is no way to determine the exchange rate of gilt to gold based on hp representing gilt as per the compendium of forgotten secrets. Hmm I was hoping the soul coins would give me an idea but that is a dead end. 

I suppose my direct issue is just needing to address “if a goblin has a max hp hp of 12 that equals 12 gilt earned if I slay him, but if I were to convert that currency to gold vis the class ability what would be a reasonable rate?”  

ive considered taking the average of all monsters hp, possibly doing it by something akin to their hp minus my class level, or even dividing it based on the 72 usable hours a soul coin gives. So for ex: a goblin has a max up of 12 so 12 \ 72 is 0.16 or one silver and six copper while a creature with 300hp / 72 is 4.16 or go gold one silver and six copper. 

I hope you don’t mind theory crafting with me a bit but you are sharp so I appreciate your insight if you are  willing 

Well, my first couple of thoughts are:

1) I haven’t read this book with gilt in it, so I’m kind of just faking and improvising;

2) Soul coins aren’t like ordinary currency, they exist to be burned up in an engine, not to stay in circulation.;

3)  Gold has no intrinsic value in the Outer Planes, beyond what you can manipulate mortals to do with it.

All that said - If you gain control over a goblin’s soul (or gilt) just by killing it, sure.  You can then trade that soul to, let’s say Mammon, for x amount of gold.  Let’s say 100 gp, for convenience.  If you gain control over a 20th level paladin’s soul, let’s say that that’s worth more to Mammon.  How much more, I don’t know, but more.  A being that wasn’t going to end up in hell anyway will be worth more than one that Mammon is just getting a few years early.  If the soul just escapes into the ether once you kill it, the paladins soul  is worthless to you. It’s in heaven playing the harp and Mammon has nothing.

If gilt is something that accrues to YOUR soul though, like you absorbed the cosmic importance of the goblin by killing it, then that affects the sale price of YOUR soul.  Let’s say as part of the terms of your warlock pact, you can forgo leveling up and instead convert XP earned into gp stuffed into a nondescript duffel bag by your patron, that’s between your patron and you, afaic.

I guess if I were looking for an actual rate of exchange, for simplicity I’d just say XP value=gp value and be done with it, but I think you need to answer some questions about who has control of the soul and how they can transfer it.

soul coin is the currency, you're asking how much is 25 cents...its 25 cents....and a quarter is worth a quarter (usually - see below). asking how much gold a soul coin is worth is like asking how many euros a dollar is worth - exchange rates I'd guess fluctuate all the time so good luck pinning that down.  all depends on when/where and how big the fee is.

 so usually a quarter is worth a quarter (same with soul coins, especially for any creature using coins as fuel), but not always. some quarters are worth way more than other quarters - which is where collectors come in.  the only question is why they'd want it - someone throwing coins into an infernal machine could care less, but you'd have to homebrew your own 'value of souls' concept in for a specific soul - and a specific person wanting that soul.   also, i'd bet coins with a B or a T on them would likely be more valuable (to collectors) than those with a Z on them.

Maybe we can unpack and start with the purpose for which you were hoping to extrapolate soul coins.  There's a Paladin of Kelemvor, who's interested in soul coins.  It'd be interesting to determine what Kelemvor's position on Hell's soul coins, but it doesn't seem like something he'd personally traffic in.  Death is nature, not a horror or a business seems to be the gist from my skim of the lore.  That said, I could see someone create some sort of Palladin who did traffic in soul bounties for those who cheat or defy death (basically a flavor of Marvel's Ghost Rider).  Is that where you're going with this?

Also this gilt is a metaphysical currency.  How does that economy work?  As mentioned it doesn't seem there's clear means of exchange between planes of existence.  I mean some extraplanar beings do hoard mortal treasures, or keep some on hand to finance dealings in mortal planes of existence, but the "natural economy" of another plane of existence and the mortal economy of precious metal coins.

Last thought, when I've been thinking of soul coins used beyond the Avernus modules, I think of them more as an easily traded commodity rather than a currency.  Like barrels of oil in the modern market.  Right now (in Descent into Avernus) soul coins are particularly valuable because Avernus is in the midst of a major Blood War incursion and uses soul coins to fuel a lot of its war machinery.  Zariel is probably doing all sorts of deals with the other planes to increase her supply.  Other ArchDevils may be supporting her, or holding her head over the barrel so to speak in dealings, and may even be hedging bets on support of her by loaning stockpiles to Tiamat and Bel on the chance Asmodeus' experiment with Zariel goes a different political direction.   

An idea I had in mind was that a soul coin's value is equal to the CR (challenge rating) of the character or soul inside; then equaling the challenge rating of a devil and their value in the blood war.

examples:

Commoner CR 0-1 = a Lemure or an Imp

Knight CR 2-5 = a bearded devil or a barbed devil
And so on 

Visually the coins all look the same but the whispers become louder or more violent representing the nature of the soul inside. The coin could also be heavier or lighter, smelling of ash, sulfur or metal, anything to help differentiate it from coins worth less or more. Devils could use the visual similarities of the coins to fool mortals who wish to own a soul coin into having a worthless penny. 

That’s a fun idea…s’not bad.  or tier rated to simplify a bit so you have 4 different values vs 34 values.