Cost of Calor Lite Gas

Last week while staying at one of the club's sites I purchased a cylinder of Calor Lite gas and was charged £28.99.  On checking with my local caravan dealer I was informed that the price of the same gas is £23.00.  I thought that being a member of the club does not mean that you will be ripped off.  Another thing, why must all purchases, apart from fees, be paid for in cash?