In my campaign world, humanoids like Goblins, Hobgoblins, Kobolds and Orcs are frequently for sale as slaves, goblins being the most common. A goblin slave (perfect for things like rat-catching, chimney sweeping, latrine washing, nighttime street sweeping etc.) costs about 100 gp. Enslaving humans, elves, dwarves or halflings is highly illegal. If it was legal, I would start prices at 500gp for a human, up to 5,000-10,000 for an elven concubine.
Strangely, my PCs have never bothered to sell any humanoids into slavery (although one of them, the halfling, has a goblin slave). Most of the players thought that actually engaging in the slave trade would be "dirty". Sodomizing a goblin with a staff to get information out of him, however, was apparently OK in their minds...