Well, a landsknecht received 4 guilders monthly, and veterans received 8. And then there was of course the battlefield booty, which I believe would make up a more important portion of larger sums of money. And their monthly expenses would take up a quarter of it.
Calculating daily expenses as two loaves of bread, a cabbage potato soup and a bottle of mead, we get an expense of 14 septims daily, basing calculations off Skyrim. Multiplying it by 30 means that a mercenary would spend about 420 septims at minimum. If that were a quarter of a mercenary's expense (Landsknecht, I believe, paid for all their expenses on the road, no free food from the company), that would mean that they would be getting about... 1680 septims monthly. Which is about 180 septims more than a Daedric War Axe.
Skyrim's economics come off as kind of fucked to me. Maybe we should use Morrowind's calculations. At least weapons can get damn expensive that way.
EDIT: Going off Morrowind's prices, daily expenses taken at two loaves of bread, one portion of saltrice and one kwama egg, it totals at about 5 septims a day, multiplied by 30 it makes it 150. If that were the quarter of our pay at minimum expenses, we would be getting about 600 septims monthly, which, by Morrowind's standards, is somewhat more reasonable. Comparably, a labourer would be getting about 1.6 guilders, which would be about 240 septims monthly by Morrowind's standards.
So, figuring our monthly pay as about 350 minus loot, since we're not Landsknechts but just a simple free company of not much strength, I think you have initiative to work out payment for a small-scale siege.