Or they would have enough money to not mind just paying for it. Sure, if you're a small caterer in L.A. and some famous movie star offers you exposure in exchange for a ten person luncheon, that's a great deal. And presumably other famous (or at least wealthy) people would be there. But if they were actually that famous and actually liked your food that much, they'd just pay you anyway. And probably still mention you online/to their friends. It would cost much to them, relatively speaking.