I often kill off all the prisoners I capture early in a battle, as these tend to be the good troops, but leave the peasants, spearmen and other 2nd rate reinforcements alive to be ransomed back

It's as much economic warfare as anything else - the apepar in an odd place, their units are all small and he has t keep paying upkeep for them.

I also rarely ransom any prisoners I lose - probably because often as not they're an enemy army I've bribed anyway lol