Ask BOC for the price for a cylinder of Nitrogen and you have your answer. after the purchase of a dive cylinder its just a couple of quid to refil it.
is the 3500 cylinder air nitrogen or CO2?
most air cylinders ballers use are either 232 bar (approx 3500 psi)
or 300 bar (approx 4500 psi) what price were you quoted?
in the end its what you want to pay that is the determining factor.