Landing fees at most GA fields are in the range £5 to £15, which is hardly a lot. I would be surprised if fuel sales alone would cover the fire truck, the runway maintenance, rates etc. Golfers pay green fees, don't they? (all I know about golf courses is that some of the fairways look handy for forced landings: better shove a tie in my flying bag so I can get a drink in the clubhouse after I've piled into the water hazard)