What always struck me as odd is that all cavalry share rather low upkeep costs in relation to infantry. At one extreme, barbarian mercenary cav is 90/turn, where skimishers are 170/turn. Shouldn't a unit of horses and men cost more to maintain than a unit of running men with throwing sticks? The simple idea would be to mod cavalry to have a higher upkeep, especially the romans, who basically would have paid a bit more for subcontracting their cavalry work to mercenaries. Of course, while a human might react to this by reworking the composition of their armies, I bet the AI would just bankrupt itself that much sooner.