I think that might be better handled by the ruler paying a lot of money of front and making the units free maintance, but have them live off the land and what they make.
Else we could just have a hidden mercenary civ from the start that advances. its who point to keep track of technology level and abundance of potential mercenaries in an area and if there are too many they can also become bandits or pirates. You might also be able to have this keep track of merchants as well.
I dunno just a thought.