"but they don't all charge £50 for camping"

No - most seem to charge between £12 and £20. But again, we're talking aobut weekend festivals. Sidmouth gives you 8 nights of camping for your £50.

"and another £24 (or £4 per day) for a bus pass"

Well, it's not a greenfield festival. A bus service helps to overcome the geographical spread of the festival, but it needs to be paid for. The bus service laid on at Towersey which takes you into town, for example, costs £1 each way, I believe.

Many people choose to come for the weekend - in which case your ticket, camping and bus pass will cost you £92 in total - easily competitive with other weekend festival tickets. If you stay all week, yes - Sidmouth does cost more than a weekend festival. But for many people who do stay all week, it's not just a festival, it's their holiday. Given the current cost of going abroad and the exchange rates this year, I think it's a particularly attractive alternative.