Not that it requires comparison, but as a captain for an airline, I am not paid until we block out (door closes) and until we block in (door opens). I spend at least an hour prior to the flight preparing for the flight and perhaps 10 minutes wrapping up the flight after the door opens. Should this have any bearing on what a flight instructor charges? I don't know except to say that in other areas of aviation there is time devoted to the flight that is unpaid. I think instructors need to be fairly compensated, but the students also need flight instructors who will charge them fairly. I think most would agree with that?