What do you DMs figure the average cost of a run of the mill slave would be? Maybe a couple average examples; human, goblin, etc.

I think prices would vary considerably according to different locations.

I can tell you about prices from the actual IG slave auctions we had, though (involving PCs buying other PCs):

Starting price was around 600, going to 6000, with most around 1100 or so.

Economy is a real difficult thing to get into though, so it's hard to say -- we reserve the right to keep prices mysterious and to fluctuate wildly.