Agreed, the Spa 24Hs are on a smaller scale than Le Mans but still worth it.
Prices:
The basic ticket is 26 Euros plus another 26 Euros for the paddock (which is advisable) and another 10 or 12 Euros for the grand stands. The grand stands are usually build up some weeks prior to the F1 GP and well worth the extra cost. Decide for yourself when you are there. Tickets are available at the gate and don't need to be booked in advance.
Camping:
Basically, you can camp all around the circuit but there's only one "camping ground" with showers and toilets which is at Les Combes. There's another camping ground close to the main grand stand in the woods which costs 10 Euros (I think). Booking is obviously not required, just turn up and pay at the gate.
Atmosphere:
The atmoshere at Spa is always relaxed and not as busy as at Le Mans. They have a fun fair at the track which is opposite the main straight but I always found it more fun to watch the race. You find beer bars and food stands all around the circuit. So, the risk of running dry is smaller than at LM. As stated before, there is no english commentry, the circuit radio is in dutch and french only.
Due to FIA F1 enforcement, there have been many spectator unfriendly changes to the circuit recently, lots of new fences which obstruct the direct view to the circuit.
Still, I can recommend a visit to the Spa 24Hs. For me, it's the second biggest event after LM.