Depends on a few factors:
Where (in the country)
When (peak times are more expensive)
What site (Farmers' fields can be dead cheap, Private or other maintained sites can be pricey - depends on what facilities they have as well - hot water, showers, a wash-up area, drying room, shop, etc etc.)
Some places are about £4 or so, others approach £20! Some charge per tent, some per person. Some add a quid or so for car parking per day.
Sites without a website are often cheaper - get their number from listings websites or by calling a local tourist info office.
Decide how much you want to rough it, how much cash you are willing to part with for what you get and take it from there.
Pack a relaxed attitude and a sense of humour and you're off to a wining start.
Oh - I'm a big fan of checking the weather forecast regularly before I go camping. recommend www.metcheck.co.uk (or is it .com ?)
Happy camping