I think that if you do make a setting where slavery is a big thing (like in Dark Sun or Al-Qadim), you should probably include prices for slaves – not necessarily for PCs to buy, but to establish what they cost. I remember that back when running Dark Sun it irked me that the only reference I had to slave prices was that one gold piece (which in other settings would be 100 gp) was considered an absolutely massive amount for one in one of the novels.In all my years of playing D&D, I've never experienced PCs who wanted to buy slaves because they were rolling in wealth. I'm sure they exist, but I don't think it's common and I don't see why WotC would include such prices in their game. Come to think of it, because the economy is so wonky, would it even make sense to to own slaves from a strictly economic point of view? The Soviets kept a bunch of German POWs after World War II to use them as forced labor but finally gave up when they figured out the program cost them far more than they would ever get back.
Now, Wizards probably isn't going to release a setting like that which would make it a moot point, but that's a different issue.