My dive club has a compressor and air is free to members, oxygen is 1p/ltr - it costs me about £5 to fill my twin 12s. Local dive shops charge between £2.50 and £4 ($4 - $7) to fill a 12ltr, more for a 15 and nitrox up to 40% is double that. Twinsets are charged as two cylinders, so potentially £16 ($25.50)to fill twin 12s with 32%.
The dive shop I used to work for didn't charge to fill a pony.
I like their top off policy, can't tell you how many times I have had my 13cf pony topped off at full price...
The dive shop I used to work for didn't charge to fill a pony.