CapnZapp
Legend
First off, if you are somehow personally offended the price of a slave isn't high (whatever that means), perhaps you should cool off? I have no stake in this.There is no "but" there. We expressly are talking about slaves at all levels, from those that are as you agree cheaper than a warhorse. To those that are worth much more. As a matter of genuine curiousity, why is it important to you that slaves are cheap? What at heart makes you resist a tier-1 PC being worth several thousand gold at market? Why is it important to deny that a skilled laborer - capable of pulling in a few gp per day - is not worth at least a thousand gp? That's something I honestly don't understand. Why do people want to push down on the prices of slaves?
Again, I'm not talking about "tier-1 PCs". A player character is off-the-charts, and I was clear about that (or at least, I thought I was).
When I'm talking goats and camels I'm talking about the utterly average.
If you have a skilled craftsman that can pull in gold per day, of course that slave is worth much more.
But a regular laborer doesn't even come close to that. I am fully assuming most people live a currency-light life, and that any services they render is only worth the printed monetary value in goods and services.
Also, D&D economies are notoriously fracked up and essentially useless.
So what do I know. Just don't make slaves so valuable that actually makes slaving a tempting alternative to more "honest" occupations. And yes, that includes plain old robbery

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In short, Drows enslave because that's what they do. No reason to even attempt to provide an economical justification for it.
High slave prices easily make for an absurd economy where people rather enslave each other than tend the crops.
It simply isn't the D&D way. Much better to keep prices so low they don't catch the attention of adventurers!
TLDR:
You asked for opinions. I gave you mine. If you don't like it you don't have to argue about it - you just don't use it. That is all.