I agree, many campsites get incredibly greedy during the summer, just because they can. Recently, I was quoted £40 a night in Dorset with a minimum booking of 3 nights. This was just for an overnight to give my daughtrer a chance to visit the outlaws, so I politely declined and we managed a day trip instead.
Mind you, right about now I'd pay almost anything to be away in the van but I'm still on visiting-from-Aus-daughter-and-partner-entertainment duty. Love having them here though and I'll get some away time next week when they head off to explore somewhere in Europe.