Sportbikes.net (same software as NESR) just implimented a
Casino as part of the forum.... they're already trying to work out the kinks but it looks like it could be promising.
Originally they had it so that you earn casino bucks through making posts, threads, rating threads etc but that quicky ended as the post whores found out
My suggestions would be to have your bank reset to 50 bucks when it reaches to zero, have it grow while you're logged in and actively browsing the forum as if earning interest, or just make $1 the lowest that your bank can go.