I'm admittedly biased against drow, but I'll take another swing
The price of a slave should be like the price of anything: supply and demand. There's surely lots of demand. Come to think of it, I bet humanoids would be more valuable than most animals in the underdark. They don't use horses to plow their fields, one would think. Don't they tend to grow lots of mushrooms and fungus and stuff? Sounds like it needs to be hand-picked, and I doubt any snotty drow would do it himself. Also, the drow seem like they'd enjoy having slaves around as sorts of symbols of wealth, like jewelry. Yes, plenty of demand.
As for supply... well, that's sorta up to you. If humans run around in scattered villages next to entrances to the underdark, and the drow raiding parties can gather slaves with no fear of retribution from military or heroes, then the supply is gonna be quite high. Your price would be well-suited for such a world where slaves can be easily harvested. However, I can't see humans being so complacent about getting kidnapped all the time. Seems like they'd fight back after a bit, send adventurers to plug the holes, set up guard posts, or something like that. I picture drow raids as being on the rarer side of things, so slaves would cost more.
The price of slaves wouldn't just depend on the frequency of raids, either. If your world has slavers on the surface, that's another source of slaves.
wingnut_dc said:
Reading through the descriptions of the Drow and their attitudes towards slaves, I would think that they would treat a horse better than a slave, especially a slave of another race. At 100gp, one would put some value in them and not kill or maim them at their pleasure as it seems they are prone to do.
True, they wouldn't go around offing their slaves, but they could certainly beat the crap out of them. Think about the terrible practices of American slaveholders. And those slaves were worth incredible sums of money. Something like $400 to $800 dollars, and that was a hell of a lot of money back then! And those slavedrivers didn't have clerics around who could heal up all those nasty wounds. So would drow kill off their slaves? No sooner than they would break their swords, set fire to their fields, or kill off those underdark-bison-things. Would they mercilessly whip, kick, and generally mistreat their slaves? Sounds good to me.
Keep in mind, that I'm not a drow fanboy, so I could be way off the mark on any of these assumptions. I was just laying out what makes sense to me
