Bless your sweet innocence Backpacker. If only gliders only cost the price of tyre wear to operate!
Don't be so condescending. Obviously there's a whole ledger full of costs that need to be paid for, eventually. I bet you haven't even named half of them. My point is just that virtually none of these costs (and actually none in your list) are related to actual airtime of the gliders. As far as the club is concerned, a 6 minute flight will cost exactly the same as a 30 minute flight. Sure, there are costs associated with the flight itself (fuel for the winch or the tug for instance) but these are per-flight costs, not per-minute costs.
How the club divides up all the annual costs that are being made, across the number of flights and number of hours flown is mostly a political matter. Some clubs charge a flat membership fee which covers all annual costs, some may charge a weekly or daily fee to casual visitors, some may charge by the flight or by the minute, or any combination thereof.