The most I've seen on domestic flights is $400-- and that was when they were pretty desperate. Once it involved the last flight out of PVD and the volunteers were to be bussed to Boston; another time it involved the last flight to Spokane, and volunteers would have had to spend the night in a hotel (courtesy of UA, of course); the other time was on a Friday night ORD-LAX flight-- they were WAY overbooked and were looking for around 15 volunteers!