I don't ever recall seeing people paying for a flight on-board but I did fly from Las Vegas to Reno (and back again later) to visit UNR and walked up to the Hughes Airwest counter, paid my $45 for a ticket, walked out to the tarmac and up the ramp and sat down on the plane. Maybe 15 minutes from parking to sitting down. The stewardess checked my ticked when I got on the plane to make sure I was on the right flight.