I wonder how much this would be worth for Zerodium
https://news.ycombinator.com/item?id=43025038
This might be a weird corner case where Apple would outbid the grey market, but generally even though Apple comes in lower than the grey market (for these very specific kinds of vulnerabilities), the term sheets are different, and the rest of the terms tend to favor going with Apple.
We recorded a podcast with Mark Dowd a year ago where he said nobody actually gets "list prices" for iOS full chain at $2.5MM (you can make considerably more than that by selling to multiple parties and by selling maintenance) --- and that's iOS, the highest-valued vulnerabilities.