I promoted to III this season with an arena at 12,500 seats (I have added 500 LT and CRT along the way this season, too).
I was earning about 210-215K at the beginning of the season and now getting closer to 230K. I can't imagine being able to push the prices enough to get an average of $20 per, though.
And we're doing well (12-6, third place), with strong Fan Surveys. I'm pretty pleased to be getting that much income. Our league average is 176K. So I'm pulling in 50K over the average without having one of the biggest arenas by a longshot. Then again, 5 teams in my league don't even have maximum receipts of 200K.
So I'd say if you promote and win at the next level, 250-260K is probably within range for you, and that's going to likely put you pretty high up in your league.