I honestly don't mate, have never rang them. I know a standard sized bottle of gas is currently £16 or £16.50 (Friend last purchased in December). I'd expect a bottle of theirs could cost you another £10 - £20 the first time only (bottle deposit or whatever you want to call it)
What I would suggest is taking along your regulator next time. Explain what it's off, and see if they have bottles which could be used with it. Failing that (and as my mate did), they suggested and sold him an alternative regulator for around £7 - again bear in mind these are one off charges. Once your in the swing of things it's just your usual £16ish for each swap-over (My mate has 2 replaced each time to make it worth the trip).
Before I suggested this place to him, we tried B&Q amongst others.... HOW MUCH!!!!?? I think they wanted about £80 the first time you take a bottle from them, and several forms of ID! We soon walked out. lol