I just paid 50 bucks for a used 20 oz. C02 cannister at a local paintball shop. You can find cheaper by checking painball forums in your area if you're willing to find and meet the seller
The remote hose for it was 59 bucks, but it doesn't regulate flow, so you have to be really careful about opening up the gas.
I checked on the price of regulators and they're 80 bucks. One of those will give you complete flow control.
I'm going to test out an alternative method where the remote hose connects to a 2 liter pop bottle. A length of hose inside the neck of the bottle will run to the bottom, and the bottle will be about half full of water. Small holes in the upper portion of the bottle will allow the gas to leak out in manageable amounts, if it works the way I hope.
So depending on how complicated you want to make your setup it can be dirt cheap, or cost you a couple of hundred bucks.