I don't think City, with all their wealth, are going to target a soon to be 33 year old. However, in the odd chance that they can't get their other targets and their CL ban isn't overturned, how much would you accept to sell him? I'm going to go with 30.
If we were a mostly complete squad and getting a top young-ish CB to replace him, I'd sell him for 20 mil. But we have quite a few other investments to make, so no way we should sell Bonucci and go into the season with an injury prone 36 year old Chiellini, a 21 year old De Ligt, and a 22 year old Demiral returning from ACL injury.
So... no to selling him. Spend our money on the midfield, the RB position, and if Higuain is leaving, a replacement forward (could even be a world class attacking winger). I'd rather not have us entirely dependent on Chiellini staying injury free and Demiral returning 100% from ACL injury to not end up in a disaster situation.