Paintballs and air are going to cost you quite a bit over time, but if you know where to look you can find some cheap ones and get going with a minimal investment.
For the balls you’re looking at about $ per 500 unless you want something specialized. It’s really not a bad deal, but as a beginner you’re likely to go through a lot of ammo until you get some accuracy and a handle on that itchy trigger finger.
Fortunately, if you’re running a gun on compressed air most fields will have air available along with the rental fee so it won’t cost you anything over the normal rental fees.
If you’re using CO2 on the other hand, or run an adapter so you can play when you don’t have an air compressor nearby you’ll be looking at about $ for 50 cartridges unless you want something specialized. Expect to get around thirty to forty shots at maximum velocity and maybe another ten at lesser speeds.
Adapters are something to look into, even for a newbie. They’re simple to install and they’ll allow you to make sure you can use the gun wherever you might be, a good but cheap adapter will run about $. Keep in mind this will void the warranty on many markers, however, and it can be hard on the internals.
Overall, for an average afternoon you’re looking at $ worth of balls and an optional $ worth of gas. It’s significantly cheaper to use compressed air for obvious reasons, and that might factor into your initial choice of marker.