I read in multiple areas it was $25m. But that's fine, was probably settled at $15m or something.
Regardless, Kentucky should be able to find the money or at least get Cal to negotiate. I know people here think Cal is some wacked clown that will go down swinging and taking Kentucky basketball with him to get that money, and all of it, but I don't agree. I want him gone and think he's definitely an egomaniac, but his core focus is that if he has to go and is forced, he will want to go in a way that doesn't leave the entire media world calling him an arrogant greedy asshole for the rest of sports history. He'll want to try to leave whatever legacy he has left in tact and go off being seen as as a compromiser, not an asshat.
I think $15-20m would do it. Cal might put up a small fight internally with making propositions to convince certain kids to return, have a complete staff overhaul, and make hollow promises to coach better, but ultimately I think those ready to pay up are fed up and don't believe a word he says. He's had his chances and still flopped. One more year with the same talent isn't going to change the outcome.