Not easy questions to answer, Mike, there are a lot of variables. You CAN spend a lot of money, but there are ways to avoid it. I'll assume you've already done some research on the rifle you want.
Some is easy, a decent hand pump will run around $250. Depending on the rifle you decide on, a regular scuba tank might work, most are 3000 psi, I believe new ones run $2-300. A carbon fiber tank of the same size can be filled to 4500 psi, that means a lot more air, they run $3-600. Depending on where you live, they can be a hassle to get filled. From what I've seen, most guys don't use regulators ( myself included), just pay attention to the fill pressure guage and go slow. If you do go with a reg, expect to pay $250-500, again depending on the rifle you select and your air source. Not needed with a pump, of course. What ever you use for an air source, you'll need a way to get air from the source to the gun. Filling assemblies vary a lot in price, and are gun specific, figure another $75 plus or minus.
Pumps can go anywhere and never need filling, but can pump moisture into your gun. And if you shoot a lot, you pump a lot :-)! Scuba tanks are heavy and bulky, and with some guns, don't offer that many fills. Some of the high pressure guns can't use a scuba tank at all. My Talon takes a 3000 psi fill, an 88 cubic foot scuba tank would only net me 6 or so fills. Since I do virtually all my shooting in my back yard, I avoided a lot of hassles and expense by renting an industrial nitrogen cylinder. 443 cubic feet at 4500 psi nets me at least 150 fills, gas was about $60 and tank rental is $10/mo. The closest dive shop is a 100 mile round trip, and they want $12 a fill. I modified a standard scuba yoke to work with the cylinder, that cost me an additional $30.
Obviously, what you spend will depend on what you shoot, and where and what you shoot at. If you're like me, and mostly punch paper in the back yard, you can get by for a lot less. My setup isn't very portable, but the Talon gives me lots of shots per fill. I plan on getting an extra tank eventually, just in case I decide I need more capacity. If I anticipated lots of time away from home, I'd probably invest in a medium size 4500 psi CF tank. Hope this helps, and if you have more questions, don't hesitate to ask. And let us know what gun you have in mind, too. Later.
Dave