I game to spend time with friends, I think money would easily taint the whole expereince. Do you pay your friends to hang out with you?
Now, theoretically, if I knew someone was an excellent GM, ran a great game and knew the rules well, I agreed with him enough in his style that I would enjoy a game of his, and he had a good setup (place to game, minis/terrain/mat ect.), I wouldn't be opposed to chipping in $5.00 per game or something if he needed it. If he was on hard financial times and GMing would be a signficiant source of income for him, but that's got to be a very special case scenario.