4's enough IMO, probably a touch more if you're taking in Spa too.
We go on the cheap now, you do it enough times, the novelty of eating out and going 'posh' wears off.
4 lads & 4 cars sharing a cabin on the Hull/Rotterdam = £90/each return
6 person apartment self catering = 15euros/pppn
Laps and fuel are your biggest cost.