Been camping outside the Spa track several times now.
There are many places to put a tent just outside the track (from a few meters to a mile or so)
The picture in the link shows you the area near La Source.
Bottom left you can see parkings that are used as campsites.
I can't remember the price per day for 2004 exactly (as I didn't pay the guy at the campsite), but it's around 10 - 15 euro per day i think.
When you drive on the road that is going around the circuit you'll see many campsites to choose from.