I think payment up front would be preferred.
"Prizes" are basically transferable field passes. That way the field owner can still sell some paint when the recipient comes back to play some more paintball. I don't think there will be enough participation to warrant anything of great value, and the more valuable the prize, the more cheating and basic punkery you get.
Teams: You hand in a roster of three player plus one alternate (who may not be a player or alternate for any other team). He does not pay until he starts playing - otherwise he ends up paying to sit on his thumbs.
__________________ "Sorry, I'm programmed to do that..."