At Dive90 they charge us £4.50 a tank (stupidly expensive...!), in which we usually get through a tank a week each, so it adds up a LOT!
But to be honest i'm not too sure about what's good when refilling with a divers tank. They use different valves, so you'll need to get a connector to link the two.
Where did you get the dive tank / paintball tank from?