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?
got the dive tank from a local diver through a friend. got it for £50. thought that was pretty ok. just needs testin as it was die in june. and have got a mate at the local fire station that might be able to fill it now and again for me. seen the fill stations on ebay aprox £15