If we really analyse it the membership fees are really very low compared to other things in life. If I fill my car with diesel it costs nearly £80 and I can only drive about 400 miles, less on Dartmoor lanes. So not too many caches to be found for £80.
If you go to a premier league football match it can cost you anything up to about £60 for one game and even Plymouth charge £20. Any club or organisation you join will charge some sort of membership fee, so maybe our fees to Groundspeak are relatively cheap.
However, they make their millions off the backs of lots of volunteers. The reviewers must spend many hours each week administering the caches and they get paid nothing. Not to mention the COs. If nobody placed any caches there would be no geocaching!