I found it surprising that a setting with slavery and slave trade being a core aspect had no tables or lists with prices for slaves. I can understand 4e wanted to take a more heroic approach (freeing the slaves, not owning them), but previous editions had no such qualms. I can also understand that such thing as slave prices was left up to the DMs to take care of. Maybe TSR feared repercussions for publishing pro-slavery stuff, especially at a time when D&D was being hounded as a satanic game?
There was a homebrewed table of slave prices, based on race, profession, stats, and class levels, if any, that circulated on the internet. I have laid eyes on it, and found it quite interesting, albeit a bit too complicated to use.