As you say, there is already an established price on "a value for each sentient life" - the 500 gp of Raise Dead.---
Back on topic:
I am certainly no expert, but I would have thought prices were much lower.
Argument #1 You're talking some kind of "fair price" where you're willing to pay upfront for what it'll take 5-10 years to recoup. I would guess nobody's looking farther away than, say, three years.
Argument #2 Look at the slaver. Look him in the eyes. Doesn't he look desperate to you. Let's give him an outrageous offer, yet one that allows him to make a small profit and return next year. Say 1/20th of what would be "fair".
Of course, in areas where slavery is dangerous or illegal, that could bring prices up. But then we're talking especially ...heinous... slave tasks. Nobody would pay through the nose for general household work - that calculation only really works where slavery is tolerated. In areas where slavery is banned, better hire a regular maid for that sort of work.
Of course D&D is a highly stratified world. It's called levels. Just because the price on a raggedy commoner slave is outrageously bad doesn't mean the price on exotic, skilled or just beautiful slaves could be much higher. Like tens of thousands times higher. After all, unless you completely reject the default "money pyramid" of D&D, high level personalities (whether NPC or PC) can afford to pay hundreds of thousands of gold on stuff they really really want. Talking djinni princes now. Depraved demon lords. Semi-retired epic adventurers.
But in general, a slave is a mundane good, and mundane goods in D&D have trivial prices.
Whatever a slave costs, it will not be comparable to magical adventuring gear. It will be comparable to whatever a goat, camel or warhorse costs.
So a wretched, old, injured, or unskilled slaves could cost as little as 1 gp (the goat level) when supply outstrips demand (I'll take all twenty, including the one with no teeth, for twenty gold and a cask of ale!")
A more attractive slave (whether through strength, education, or "personality") could go for 50 gp (the camel level) or, of course, some multiple of it if there's a bidding war.
A spectacular slave (maybe a trained gladiator, a slave Princess, or an otherwise exotic specimen) could have a starting bidding price of 400 gp (the warhorse level).
Then a captured angel, or archmage, could go for much, much more. But then we're really not talking commodity slaves anylonger.
Whatever you end up with, make sure prices aren't fair or equal. Especially in a D&D world.
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