If you decide to use entry fees for prizes then we suggest you announce what achievements will earn prizes but not exactly what that prize will be. You can decide the full extent of the prizes once you've collected the entry fees. Most stores in the United States charge something like $15 for each player for a multi-round, single-day event.
But estimate your expected attendance on the lower side so you can cover costs. If your event fills up to maximum capacity and you end up with extra cash after awarding prizes you can always offer to buy pizza for everyone!