Would you pay a GM?
I would, especially for games that are very hard to find GMs for like M&M.
I offered once, at some other (undisclosed) site, but unfortunately there were no takers.
As for fees, I suppose that depends. I think if I am going to be paying a person to run, I would want them to get compensated fairly well so they feel they can put the energy and work into a good game. I'd say $20 - $30 a person, so that a GM running a four hour session with four people can turn in a $20 to $30 hour earnings. Better if there's more players, or it just runs for three hours.
I could afford that once a week. I would see it as a "Hey, this is awesome" tip. =)
And if the GM had enough groups to play the same scenario once a week, say three or four, it might turn out be a nice side gig. But I don't think they should quit their day jobs.