What about a high game payout?
I was in a league a few years ago that did that. Basically you paid $5 (maybe it was $10) to get in. After the first game they eliminated the bottom 1/3, then the 2nd game they eliminate the bottom 1/2, then 3rd game they pay the top 3.
High score from each game would get their money back, even if you were already eliminated in the previous round.
That always seemed to have a higher participation than the brackets - you always seem to have those guys that are good for 1 high game a night, so they figure they have a chance of at least getting their money back.