His last deal running to 2023 then makes even more sense for a release figure acceptable to both parties to be agreed with a new 2 year extension he signed.
If 75m was the supposed benchmark utd would have had to pay (but wouldn't because he wasnt worth that), the following year that becomes less and his desire to leave possibly more.
Agreeing a release clause around the same amount or marginally higher is the only logical option. Lower, makes no sense for villa. Higher, no sense for Grealish. There or thereabouts, Villa get to safeguard that value, the player gets to keep his options open while getting higher wages and showing respect to his club and fans.
I've been a sceptic of a release clause all along, but I'm getting more and more to it. Particularly as noted before, the way Villa have treated this, making it look like there is one. My guess is around the 80m mark.