Bit late to the game, but there's one option not really mentioned yet; the foreign 'volunteer'.

The [monster] tribe of X has agreed to 'submit' to a nearby ruling [civilised] group. It's simple; Y amount of [monsters] will be offered as mercenaries per year, and they will behave otherwise the [civilised] will march up and destroy them all for 'breaking the agreement'. The [monsters] have learned from experience that the [civilised] ain't bluffing, and in fact the [monster] elders have come to realise that it's actually not that bad a deal [safety valve etc]. If the 'agreement' is a long-standing one, it might even be part of their society; that a teenage [monster] goes off 'outside', and after a few seasons some of them return - bigger, stronger and with some nifty 'outside' things which immediately jumps them up the [monster] pecking order.

Quote Originally Posted by Martin Greywolf View Post
...Last but not least - not least by a long shot - is loot. Even regular soldiers were underpaid in medieval times during campaigns, with the understanding that they will be able to take loot, whether more traditional form, or by capturing rich prisoners and ransoming them (though onyl a noble can ransom a noble). This was a substantial increase in income on a successful campaign, especially since an army has to send smaller detachments to secure food and water - who will stop you if you also loot some coin from the farmers?...
Which is why I have my PC in a Mage the Ascension game who always search corpses for wallets, gathers up the weapons and steals any car going. After all, not only do they not get paid they have to source their own kit...