I'm planning to do something similar in Anno Domini but without using mercenaries but having different "feudal" units with very low cost of raising and upkeeping... until they stay into settlements. When they move around the player will have to pay lot of money following a coherent system (it is already done but need tweaking and playtesting).

Unfortunately this huge modifications will have to be done for the player only as AI is unable to disband and to manage things efficiently, so they will just have some cheap troops (and we all know how much it is needed due to the poor selection usually AI put in the field anyway).