Pay to play is, as noted, common at conventions.
At tables its rare, but i have met a few pro gms. Heck i have been bribed to run games before (less cash, more snacks. But this was whole 12 packs of soda and the like from multi players.) And ran horror games for clarity.

Pay for play is fine, as long as the gm and players are cool with it. Gming is a skill, and if you have players who need a gm, its not a bad option. And removes some issues. The gm is likly to be very fair, not favor anyone for past friendships, no gm "partner/player" issues and if they are getting payed they will be on time and ready to go. :)

Now would i pay a gm? Sure, not sure how much, but 10/20 for a good session seems ok. More if its an all day thing, or meals/props are included.