A point has been raised in the main XGM forum about Barbarian units.
Historically, barbarians would not have standing armies, they would levy an army quickly, but that army would also disband quickly, especially if it wasn't doing anything.
Now this is hard to model with the RTW engine, but one suggestion is to raise the upkeep costs for barbarian units. This would entail lowering the recruitment costs though.
Overall, barbarian units would cost less to recruit than civilization units, but they would have higher upkeep costs.