That's should be okay in some cases that cavs are strong, it's even realistic. But if they are SO strong why not to increase their PRICE to make it COMPARABLE to their POWER?
It terms of balance cavalry has incredible kill pressure, they are good in capturing different points due to their high mobility, they have extremely low risk - high reward due to length of their spears and high mobility (infantry can't hit you first with a spear - only with a pike, but not just every troop, not every faction have them -, only your horse. So you can get damage, but ride away in most sticky situations like this). And all this things apply to light cavalry aswell. While it's even cheaper than some infantry.
And even in terms of realistic horses aren't any cheap, especially in those ages. And you have not only to buy horses and equipment to cavalry, you have to train this men also.
