Well, it's actually quite easy to see how much it costs for a nation to produce slaves, since it's a straight gold to slaves conversion.
The cost of a blood slave = Income of the province you're hunting in + upkeep cost of the hunters / the average number of captured blood slaves.
Obviously this means that the cost depends on the blood hunters you have available, but for regular blood nations the bulk of the cost is in the income loss.
For myself - without opening the game and start doing the math - I'm guessing that it's costing me ~7g or a bit more to capture one blood slave, though that might quickly go up to maybe 10 once I start bloodhunting in less optimal (ie. population 7000+ ) provinces. There's obviously also the opportunity cost of enabling a potential rival to start in on a blood economy, but that's another issue.
Convert to gems depending on how much you value gold.
All of the above was just theoretical info to help you establish a fair price. Of course, if you were to ask me what
I would ask, I might ask an unfair price instead.
(Hint: I'm still short of earth gems.
)
More seriously, I'm currently not in any great rush to sell slaves, as my income atm is quite modest, and I'm still busy building up my blood economy. Selling slaves at this stage would mean having to buy extra blood hunters while I also want to recruit researchers and battle mages. However, if you want to do business, PM me. If you come up with an intresting offer we'll talk. (And I will actually do the above math for real.
)
Edit: Weee! New rank.