A few decades back, a friend of mine and I figured $20 million would do it. With that we could do pretty much anything "we" really wanted to do, not counting silliness.
I don't have $20 million today, but I can do whatever we really want to do, so long as I don't go crazy. If I had more, I'd be looking at giving more away.
I think buying a plane is worse than a boat. I had shared owndership once and was the Treasurer, so I know how much they cost, even little ones.