In my experience the most I have paid for a night, except for a notable few exceptions, (I stiil gnash my teeth thinking about them), is up to £10. You will always come across sites that are rip offs, difficult to know where they are if like me you dont make an effort to check in advance. As I've experienced it the most expensive sites for one man tent campers tend to be in the South of the UK, other Forum members might disagree?
In France I have paid the equuvelant of £3.50 more than once, I especially like the Municiple sites, great idea. But even here you will always come across the high chargers, I find they are often the sites where static campers tend to congregate.
If you can, do a bit of wild camping to lesson the overall cost and try hitchiking when needed, that saves money as well. Railways in this country are not cheap.
Thats what I'm planning to do more and more.