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.