Very few acts and in particular good up and coming Indy rock acts have that type of money. And if their label or management pops for their " entry fee" it is simply a loan. Monster rock acts, dinosaurs or otherwise can care less. Their fan bases are secure in some cases for decades. No acts or very, very, few make anything significant from recording these days. So where or what is the benefit for paying to be on the tube, heavily censored for 12 minutes?
so as long as the people who actually watch the halftime show can handle Rihanna, Jay Z or Madonna year after year. This will backfire. The greed of the NFL will bite them someday. The sooner the better. Some acts cost well North of a million to even stage a show. This is American corporate greed at it's best.