That really depends on where you buy your paint and what kind of paint you're used to buying :-D
If you don't mind cheap paint and you're able to drive around a bit, find a store like warped sportz (
www.warpedsportzla.com) and they will have paint for something along the lines of $35-40 a case for house paint. If you want to shoot Evil or paint like that it's going to be more like $60 a case. Entry to most fields is $20 or so. At SC it's $10 for the Arenas (the tournament fields, about 3 hyperball and a couple airball fields) and another $10 for all day air. All in all, you're looking at between $55 and 80 for a day, possibly less or more depending on how many times you like to pull the trigger, which isn't that bad really. Buying paint at the field usually isn't a good idea, most places charge far too much for it. The field I play at most of the time sells me house paint for $40, but that's only because I have a year long membership, it's $80 a case otherwise
Tell me where you're going and I can maybe find out where you should go for paint and everything.