Which black market? Who is buying it? The reason they quote such a huge range of prices is that there is a huge range of utility across different exploits, and many of them aren't worth much at all, including some that seem ultra-powerful on the tin.
Keep in mind also that the economics of bug bounties are different than those of the "black market". Bounties quote lower prices because they're offering assured payouts, often with lower exploit proof and enablement requirements. They're not actually apples and oranges.
