Would depend on the game and community. I came from the Socom community where there were more people, let alone adults.(Game was rated M, so it wasn't as easy for kids to get the game and play it) Quite a few of them were very competitive and wouldn't mind paying money for a tournament. Some people played the series so much, and were very good, that they figured why not make a few $(could be hundreds or thousands to the winning team, not to mention 2nd and 3rd place.) while playing the game they love?
One of the groups that help set up tournaments got so big that they grew from just being Socom. I am pretty sure now that it is so big, people don't need to donate and advertisement pays for it but I could be wrong on that. Haven't checked on it in a while, and I haven't been in that part of that competitive community for a very long time.
I am referring to Socombattles, AKA Gamebattles.
I keep thinking that there was 2nd group like gamebattles, but I cant remember it.