Intercasino’s tournaments seem to have been pretty successful at their price point of $25 entry fees. If you can draw 100 entries, that generates a prize pool of $2500, allowing a first place prize of $1000 to $1500, with no casino money added.
I would also consider ‘comping’ entries with enough regular play. The goal is to get some table play out of the contest, and that’s one way to get players thinking along those lines. Maybe, free entry for all players with more than $10K in action?
Holding a one-shot event is probably not going to be that successful, but if you let players know up front that you’ll be holding these events once or twice a month, I would expect you to generate a following over time.
Rather than making the prize pool dependent on entries, I would encourage you to go out on a limb and guarantee the prize money. In land-based tournaments, I’ve seen it difficult to build up repeat events unless the prize pool was guaranteed.
Of course, as a player, I’d love to see a 100% return of entry fees, plus a minimum guarantee. 
Hope these ideas help, -Ken-