It totally depends on the site. The site I have just got back from was £5 pppn (per person per night), but others will charge in various ways - some I can think of are as follows:
Per pitch + extras - This is normally given as £xx per pitch per night (a pitch includeds x adults and x children. Further adults / children / dogs etc are an additional £x per night
Per pitch based on size of tent - either by dimensions or by person up to 3 man small tent £x per night, 4-5 man £x per night, 6 + man £ x per night. some of these will be broken down even further.
There's obviously the with / without electric hookup options (EHU - you'll need your own leads and EHU unit for your tent).
Some sites will have charges for kitchen tents / pup tents / gazebos etc and for additional cars etc.
Depending on the site some will have super size pitches, which will be extra - and there's also sometimes low - mid - high season to look out for sometimes, too...
The easiest thing to do if you're unsure is to send them an email or ring them, It could be useful to have the dimensions of your tent available in case they ask for them.
Good luck - You'll get the hang of it soon enough!