I voted $12-$15/hour, but that makes four assumptions.
One, I'm running the game (and living in) the United States. Rates in other countries would vary by cost of living.
Two, that I'm only being paid for the time I'm actually running the game. Prep time is unpaid. If prep time is paid (and fairly clocked somehow), I could see going as low as $10.00/hour or perhaps closer to minimum wage.
Three, there is a measure of competition from other professional GMs. So, skipping prep entirely (unless you're a heck of a on-the-fly GM) is not just unprofessional, it puts you out of work. I personally wouldn't skimp on prep, but as a general rule this has to be stated.
Four, I am running the game with above-average skill and with a professional attitude. That means, I do not skip pre-scheduled sessions except in true emergencies (life in immediate danger, completely incapacitated and unable to reach site of game, wife about to give birth, etc.). It also means I run the game without complaint, regardless of the players' attitude toward it - whether they're friendly or jerks, involved or constantly chatting out of character and rolling dice on their turn. It means I arbitrate disputes fairly and impartially without fail. It means I do the best job I can providing the style of game the players want, be it hack and slash or deep immersion. Finally, it means I come to the sessions fully prepared and with an upbeat attitude, regardless of my actual feeling about the game.
I would both chip in (say, $3.00-$3.50/hour each from four players) for a GM who lived up to those standards, and would run games for the same rate.