Sooty, we stayed at the municipal campsite called Eau Rouge last year, it's just off the old circuit and near to the Karting entrance to the circuit. I can't remember the cost but it wasn't expensive. We've just booked to go there again. http://www.eaurouge.nl/index_en.htm
We don't need electric hook up and were on what I guess is an overflow field next to the childrens playground and along the side of the river. It was beautiful, we camped from early in the week when it was pretty quiet, just us, a Belgium family, a dutch father and son and some Danes (and a very young couple who amused us for some time trying to put their backpacking tent up until Mr Belgique took pity on them and helped!) We had a fab time - they have fresh bread and croissant delivered in the morning and there was a bar. It was really friendly.
We left on the Friday night to go to a hotel and it was starting to fill up with Brits and the character of it changed. We're camping for the race weekend this time.
We had a media parking pass so drove to the circuit but the Danes had bikes and it wasn't too far to cycle and this avoiding the parking charge and any traffic.
I think you can camp on the circuit, but I think the camping areas are pretty rough and ready with no or little facilities.