A few years ago I had a tenner top limit, it's now double that and sometimes I'm still struggling to find somewhere. Camped twice so far this year, £9pn and £14pn, another booked for later this month is £18pn, a site I regularly used to stay is now glamping only from £35pn with a minimum of 2 nights.
Must have been a tough few years for campsites, if they're now finding people will pay those prices, good luck to them, but two other trips I have planned are now Travelodge and Premier Inn for a few quid more. I think many sites are licensed X pitches, if so it doesn't seem unreasonable to charge similar prices regardless of how you got there.