From what I'm told, my mom works at a company that does payroll for the entertainment industry, Rourke wasn't even payed for the role. Aronofsky wanted him so bad that the backers told him they would put up the money if Rourke was in it over Cage. Aronofsy contacted Rourke and told him he couldn't pay him, but he'd get him an Oscar. At first Rourke didn't want to do it, but after reading the script, he jumped on it.
I haven't seen MILK or BENJAMIN BUTTON, but Rourke did the best job I have seen all year, and feel if Cage was in the role, the movie wouldn't be anywhere near as good as it was.