Playing as the ERE, I find myself constantly checking the mercenary market. I disbanded all my scout equitae on the first turn and have been steadily replacing them with cheaper, stronger local options. I would also disband my mediocre archers were it not for the fact that there's never enough archer mercenaries to replace them.

I think the decision to lower the upkeep costs for mercenaries to be in line with regulars is one of the best made by CA for this game.
This allows us to create armies as they were in that period, multinational forces were contingents from barbarian sources often fighting in lands distant from their native ones.