Most folks have hit on the crux of the matter. Is paintball expensive? It depends.
For one weekend a month, with some pretty basic kit, you'll have a blast for cheap.
Playing tournaments and practicing every single weekend, traveling all over the states, and having events every 3 weeks...cost me a TON. Like 20 grand USD over 20 months or so. That was with not a lot of sponsors.
When I had a full ride from Empire, all I spent was hotel/rental car/flights. That's still expensive but not that bad, as it meant free entry to NPPL events, free paint (OMG the feeling of walking up to a semi truck of paint and asking for 20 cases for free is awesome). All gear was below cost from a local store/damn near free.
You have to spent a lot of time, money, wear and tear on your body.
Is paintball expensive?
It depends.