Cav should be more expensive in upkeep, RTW encourages cav rich armies that don't fit historical profiles. (Those awesome Macedonian light lancers come to mind, they come in nearly full stacks.) The horses we have in RTW seem quite powerful...but even if we ignore that, there is going to be considerable upkeep cost. A horse maintained for war is not going to be cheap. You have boarding costs as well as food and water and gear to worry about. Horses on the move are not going to have time to graze. I imagine a horse would have been every bit as expensive to maintain as an individual soldier, if not twice as much.

US Civil War cavalry were very difficult to support and the land around cav forces was often stripped clean. Frequently there was little or nothing to feed them and a great many were "broken down" by overuse and lack of food and care. Lincoln, disheartened at the loss of sorely needed horses said, "I can make more generals, but horses cost money."