Good day all,

    Is there a good resource/reference for guidelines on how much mercenaries should cost?

Here is my hypothetical.  Let's say you are the ruler of a small town and the players arrive.

You could say, "I can get 10 footsoldiers from Waterdeep here in 2 days, but that will cost me XXX.  But you look like a hardened band of adventurers, I'll pay you that sum instead if you can clear out the orcs that have been plaguing the main road."

If a town, or even the players (in preparation for a large assault) look to hire mercenaries,  what should the costs be, and what would go into that decision.

I can think of things like: skill level, duration, quantity, high risk pay etc.

The closest reference I could find was in the DMG which stated about 2 gold per day  for a level 0 (CR 1/8) fighter.

Thanks!

Let's ask a simple question, "What kind of lifestyle do the mercenaries have?"

LIFESTYLE | PRICE / DAY
Wealthy | 4 gp
Comfortable | 2 gp
Modest | 1 gp
Poor | 2 sp
Squalid | 1 sp

This can give us a lot of flavor for how the mercenaries in question are. For instance, let's say we have Wealthy Mercenaries. What does that look like? Probably they are professional fighters; perhaps a group of well-armored knights or an exotic band of cunning duelists from a distant land. (CR 1; high loyalty, there's only a 10% chance they'd run away from the dragon fight.)

Comfortable mercenaries are the more standard stock, veteran soldiers of fortune skilled enough with a weapon and equipped with regular armor. They'd likely belong to a larger company, easy to find. (CR 1/2; normal loyalty, there's a 40% chance they'd just run away from the dragon fight.)

Modest mercenaries might be more in the realm of thieves or fledgling adventurers. They can handle themselves in a fight, but wouldn't risk life or limb for what they're paid. (CR 1/4 ; modest loyalty, there's a 60% chance they'd just run away from the dragon fight.)  

Squalid mercenaries belong to a violent society, though they are not inherently suited for fighting. This type might keep a sword or spear nearby, but has only ever had to use it once to scare away some wolves. (CR 1/8; low loyalty , there's an 80% chance they'd just run away from the dragon fight.) 

Poor mercenaries are common folk, probably desperate for some coin or not needing much motivation to fight despite their absolute lack of skill in combat. (CR 0; no loyalty, there's a 90% chance they'd run away from the dragon fight.)

So how do we pay these mercenaries? I think that the customers are paying for the daily lifestyle of the mercenaries as the chart above shows. However, the mercenaries might offer their services strictly by contract (especially if they're professionals). To follow your earlier idea:

"I can get ten Bloodaxes here in 2 days, but that'll cost me 60 gold pieces.  But you look like a hardened band of adventurers, I'll pay you that sum instead if you can clear out the orcs that have been plaguing the main road." In this hypothetical, The Bloodaxes are a company of comfortable mercenaries. For a group of 10 mercs she'd have to pay each of them their 2gp/day wage and she'd need them for at least 3 days, assuming they could deal with the problem in a single day. 

Obviously hiring, arming, and maintaining an active legion or unit of mercenaries is complex, but for a standard "how much would it cost" situation, I think this is a good way to establish it. 

If you can dig it up, the AD&D DMG had tables for hirelings, craftsman, and servants. Hirelings (mercenaries with no immediate ties except pay) ranged from 1sp per day (squires) to 5gp per day (heavy infantry) and their upkeep did not include extra expenses such as new equipment or mounts. In the PC ruleset, though, it was the only way for some classes to survive if you were either running solo or with a very small, tank/heal-light group.

I will admit my early D&D experience was rather sad as I didn't have many people locally I could canpaign with. Instead, I tended to playtest a lot of dungeons and builds solo. A good way to keep a mage alive in AD&D was to get some spearmen (cheapest hirelings) because mages were easy to hit and only had 1 spell uae before they were just a nussance. Mages also had a respectable amount of starting gold without the need to purchase equipment, so a lvl 1 would be able to hire 6 spearmen for 1-2 weeks at a time.

One thought with your assessment. You have to consider the life and care of the mercenary within the company.  For example, you could pay a mercenary 1sp per day, but include the cost food, weapons, lodging and training above that.  Therefore, the company may hire out the individual at a value of 1 or 2 pg per day (more for higher ranking), but the individual mercenary would see about 1/10th or so of that, per day, as their pay.  Just a thought...