Hi
Wasn't sure of the kid of price you were hoping to pay - however a few sites which come to mind are:
Nunnington Farm in West Wittering and
Brakes Coppice, Battle near Hastings. I have to say a lot of the sites near to the beach tend to be the big, all singing type of sites and therefore charge quite a bit for their pitches. Go 10-15 miles inland and it gets a bit better. Another cracker is Honeybridge Park Herebut but that's about half an hour from the beach.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets