Hey TP, if you want to keep the deal up for longer you could do something like this...
Once someone gets to 250 posts, they get to play you (or a moderator or something) heads-up. If they win, then they get the $25. If they lose, then they've got to keep working to 500 to get another shot
Would be fun, because it would add some more poker into our site and make it cheaper for you.