I can see him leaving for the UFC simply to have another go of it without the diverticulitis hampering him, the thing is though the upper echalon of fighters; JDS, Cain, Werdum are simply too good for Brock, he'd probably smash the rest of them though in all honesty. At the same time he already won the UFC Title, he beat guys like Couture, Mir and Carwin, so in a shallow as hell HW division whats there left to do? Of course though the UFC have no big money draws thesedays; Weidman, Rousey, Jones do alright, the rest of them do bugger all PPV buys, even the current HW Champ; Cain can barely crack a few hundred thousand buys because he has zero personality and tends to win ugly.
Dana would probably pay Brock whatever he asked for. Theres a reason that in the UFC someone like Connor McGregar is getting so much hype and thats because they need big stars.