$15M (e.g.
https://twitter.com/FOS/status/1768718493342404921 )
I searched Twitter and that's the only figure I've seen reported.
And, again, if Stackhouse had an "auto-extension" clause in his contract, then the strategy of "ramping up NIL until the clock runs out on the contract" isn't an option, because the increased NIL support would likely yield enough wins to trigger the auto-extension, preventing the buyout from decreasing.