Don't really agree with the idea of different prices for different prisoners, after all they are all going to end up as galley slaves, and one man's labour is as good as another's.
However as I said before I don't see the point in having different "categories" of prisoners, ie bandits, mountain bandits, river pirates etc. Prisoners are prisoners, what's the point in having them categorized in this way when they all end up as galley slaves anyway?
I guess what you could have are special categories of prisoners that bring more money, say, bandit chiefs who have a price on their heads, enemy lords and so on. In fact, you might be able to take on quests to capture or kill these folks as a means of laying your hands on some worthwhile cash.
But regardless of that I still think the price of prisoners should be jacked up, to say 1000 gold per prisoner (with a bit of variation depending on condition) instead of a measly 100. I'm inclined to think that prisoner trading should be a major means of earning money in this game rather than mere battlefield loot. Getting money from battles is just too easy. You take prisoners, then you have to safely get them to a slave trader to get your dough. That not only forces you to travel around more, so you can't just bash heads/cash loot at nearest town/repeat, but it also means a chance you might get waylaid by a stronger party and have to hand over your precious prisoners or have them stolen from you.