Oh it is, a warhorse is very expensive. But they were still expected to take multiple warhorses into a battle in case one of them got killed. And fighting your way back is easier said than done.

The most common reason for loosing a warhorse is probably not that it was killed. It was that it was wounded by archers. In that case you can ride back, swap horses and join the cavalry again. And tend to the wounded horse after the battle.