The D&D economy is utter nonsense, granted. However, we must make use of the tools which are availed to us. So, my fellow D&D likers, how much do you figure it would cost to hire a company of mercenaries, let's say, per month?

I haven't looked into the numbers yet, but I am about to start. I'd be interested to hear your estimations as well, and, well, better if we arrive at that point at the same time. 

Depends on the job and how many mercs in the company.

My preliminary estimations are about 1300 gp a month.

Using the hireling rates from the players handbook. If we assume about 400 soldiers with 25 officers (surgeons, quartermasters, cooks, sergeants and other, mostly, non-combat administrative personnel), then we arrive at 500 for the officers and 800 for the soldiers. 

I'm curious how such wages stack up against the costs of good in the economy. Then there are operation costs for a military organisation: replacement weapons, rations and provisions, accommodation. I suspect basic wages does not incorporate. 

Maybe also 400 men would be too large for the societies we see in this world. After all you would need at least a small city to house that many people. Even if you quartered them in houses in town, 400 people would outstrip most communities as presented in D&D. 

My guess, based on the operating costs of a company of condottieri in Medieval Italy, would be about 5000 GP/monthly for a company of 200-250 men.  In half-assedly arriving at this guess, I assume the Lombardy scudo = 1gp.  The cost would vary based on how good the company was and what they were being asked to do (or NOT do, as in "don't pillage from my peasants, I don't need the headache").  The existence of magic-users throws everything out of whack, of course.

I don't believe you could hire such a group by the month, though.  And mercenaries are, well, mercenary.  

The nature of the assignment and the quality and size of the mercenaries you're hiring are going to heavily influence their prices.

I'd factor in not only game mechanics, but also culture in the world you've built/are playing in. 

For instance: 

(a) How much do the mercs like gold? Is there a super awesome mercenary unit who may be hired based on some commodity that these persons want/need? Some hiring is, after all, done through trade, rather than just cash. 

(b) What's motivating the mercenaries? If they're nomads driven from their homeland, they may be ferocious fighters who can be hired for just enough gold to put food on their table. That would make their price cheap for a great outcome. But...what if they're privileged pooches who spent all their free time training to become professional killers? Than you're forking out a lot of gold, but in exchange you have the best that money can buy, satisfaction (most likely) guaranteed. 

(c) You could go with what you pay for. Less money, less skill and AC/HP; more money, more punching power. 

Now, as to how much gold? Think about it. Think about how much the hiring agent will give to have mercenaries do their dirty work. 

Want to follow up with 6thLyranGuard's point that there's question of gear (or are you outfitting the unit) and quality of mercenary.  Training, discipline, and experience are all factors to consider when settling your rate.

Something else to think over is the nature of this call to arms.  If this is the PCs land being defended, or if this is the PCs taking their lands to war, you could have a Yeomanry or militia which would likely make it less expensive since you've been paying for training and maintenance over time already.  

The other thing to think of is other terms besides set pay.  If this is a campaign of conquest or maybe even defense and you possess huge tracts of undeveloped land, you could grant titles or freeholds to the mercenary's for making it through to the end.  Maybe hire in addition to a more traditional formation, a unit of skirmishes or raiders who basically have a land based equivalent of a Letter of Marque.

Out of curiosity, has anyone every contracted out elements with the Underdark for sapper duties?  Like literally bring your enemy's power base down, very far down.

I'm 100% certain, should you ask this question on the rpg.net forums, one of the many brilliant geeks that have spent 3 decades studying and learning about 'medieval mercenary costs' will chime in and provide fuel to get you up and running.

Note; mercenaries primarily work for loot. The deal is, whomever had hired them will look the other way when they pillage. 

Pillaging after battle was standard among armies of all types for thousands of years.  Mercenaries would still expect to get paid in addition to that.

Yes, you're right. What I'm saying is that they were "given" such over the regular soldiery. 

And depending on the setting, armies in the past did NOT always pillage. In the middle ages it wasn't always so, kings paid a lot to have lords and their knights fight for them. For example. 

There are many parameters I would consider if I were a DM setting this cost.  You answered many questions when you provided your example.  I thought your question was more attuned to hiring leveled NPC mercenaries, but you are just talking about a band of soldiers, not Rat Catchers.

The First question is: How many?

The second question is: For how long?

The third question is: What level?

The fourth question is: Do they get a share of the loot?

The fifth question is: Will you be paying "expenses?"

And the sixth question is:  Will you be throwing in any equipment (& can keep it)?

The last question, for astute Rat Catchers is: And what do you need us to do?

Now in your case, you want to hire an actual mercenary or military company, well two it seems.  I can't for the life of me figure out what you plan to do with 400 soldiers, but that's your deal.  For a negotiation like this, I would expect you to negotiate with the top two or three officers for the services of the whole bunch, and they would deal with the individual soldiers.  Each company, or about 200 soldiers would include 6 officers, eight senior sergeants, and about forty junior sergeants, some of which would be your skilled labor such as an armorer.

Interesting question.  I hope it works out for you.

Of course, that was just as often due to their employer trying to cheat them out of their pay, either by getting cute when it came to handing out the gold or simply by deliberately throwing them into the jaws of the enemy to get ground up.

5gp / day is what I use for the Znir Pact Gnolls in my Eberron game, modified by if they can loot the battlefield or not. But that is for hiring a small amount of them (less than 10).