Alright, let me take a wee crack at this; tell me what you think:

There are 8 armors that can be made from Adamantine and Mithral.  If the RAW says there value range is +101 gp - +500 gp, then chain shirt would cost +101 gp and Field plate will cost +500: and the other 6 in between should cost an incremental price between 101 and 500 based on weight.  some of them share a weight and the first two have a lesser heft than the last 6; so I am thinking:

Chain Shirt 50gp + 101 gp = 150 gp

Breastplate 400 gp + 101 gp = 500 gp

Halfplate 750 gp + 200 gp = 950 gp

Ringmail 30gp + 200 gp = 230 gp

Scalemail 50gp + 275 gp = 325 gp

Chainmail 75gp + +350 gp = 425 gp

Splintmail 200 gp + 425 gp = 625 gp

Fieldplate 1,500gp + 500 gp = 2000 gp

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Now all that being said, I don't know that I agree with such a small price.  I disagree also with that poster who was saying it should be rarer than the rarest gems and more valueable than crown jewels etc, (-it shouldn't cost any more than the value of the shire of hobbiton...)  but I do think it should cost more like +100-500 gp per pound (of the item) rather than per item such that a mithral shirt would cost 1,050 gp rather than 150gp and an adamantine shirt would cost 2,050 gp (mithral halves item weight).

In storm king’s thunder, a 1000lb fragment of the Vonindod costs 15000… that doesn’t seem right. Shouldn’t it be closer to 100000?

And one pound of gold is 50gp. 1000 pounds of gold would be 50,000gp, making adamantine a measly 15gp per pound.

Quote from YoichiDragonforged >>

In storm king’s thunder, a 1000lb fragment of the Vonindod costs 15000… that doesn’t seem right. Shouldn’t it be closer to 100000?

just to explain it away...I'd say its a combination of not being pure adamantine and the local market: "Meh, everyone these days has a chunk of Vonindod they're trying to get rid of"

The reason adamantine shows up as an Uncommon Magic Item is because you add the uncommon magic item price to the price of the standard armor. 

Yes, that's a fair system. It is the basis of the system I use.

Yes, that applies to mithril armor as well.

Keep in mind that this is the base price. Adjust accordingly based on the setting. Mithril armor in Mithril Hall should logically be cheaper to acquire than it would on a remote island where metal needs to be imported. If there is a war going on then armor is going to be in more demand so the price would logically go up. Since I'm usually not tracking things like that I just go with a simply 1d6 or 1d10 roll, multiply by 100 and then add that to the price of the standard armor to determine if it's overstocked or in high demand. 

I have neither mithral nor adamant, so I don’t stress it.

now, the orichalcum market…

Ok, but why would some treasure I wanted, randomly end up in a treasure trove? That doesn't seem realistic, conceptually. You want a vorpal sword?!? Whaalla! Here it is in this treasure chest in the next dungeon... almost as if Google was listening and knew what adds to put in front of you. Poppycock.

Quote from BlamelesstheBlamed >>

Ok, but why would some treasure I wanted, randomly end up in a treasure trove? That doesn't seem realistic, conceptually. You want a vorpal sword?!? Whaalla! Here it is in this treasure chest in the next dungeon... almost as if Google was listening and knew what adds to put in front of you. Poppycock.

 That's really more a failure of perspective. You may want a Vorpal Sword, but your character, the wide-eyed adventuring youth doesn't know what a the legendary rarity Vorpal Sword is,  or if they do, it's a sword of legend that one might go on a quest for and possibly find in some lost treasure trove as a reward for their long adventuring journey. But assuming that they only know of a legend about a very sharp sword found in tales of old, maybe you (the player) have your eye on a very rare Sword of Sharpness, but your character is just looking for a cool magical sword and, if they happen to find one in a treasure hoard, then that just happens to be the treasure they gravitate towards. The DM is just doing their part to give you (the player) what you want out of the game and you (the character) rewards that befit their adventuring effort. 

Random loot sometimes leads to interesting and emergent gameplay and character development, but just as often (or more often than not), it also can lead to players and characters not caring about the treasure they find and wasting narrative capital to trade it away for what they wanted in the first place. 

some thoughts from the peanut gallery
1) when you look at the properties as we normally think of them then Mithril and Admantine have some real world analogs:
Mithril = titanium - much lighter than steel (about 1/3), as strong as steel, corrosion resistant and heat resistant. working titanium/mithril calls for significantly higher temperatures ( at least as much higher than steel as steel's temperatures are higher than bronze. and only a few "smiths" in the world really know how to work with titanium.
Admantine - super hard and dense (heavy), resistant to impact, resistant to corrosion, and heat - welcome to Tungsten. it can be alloyed with steel to make a strong tough steels but calls for higher temperatures and greater skills to work. Or, it can be worked alone - even denser, harder, temperature and corrosion resistant and difficult to work requiring extremely high temperatures (above those of titanium).  

unless you're trying to create a functional economic system for your world I wouldn't go much beyond the +500 GP to 3x normal prices for items made of these metals its just not worth it game time wise.

Sorry AEDorsay but your "Orichalcum" real world analog is simply  - brass an alloy of copper and zinc. That is what the various "orichalcum" bars from roman and earlier wrecks are. 

in the spoiler you will find some various types of brass used to day with the rough percents of copper and zinc: