This is a common dilemma, we've stopped on both sides of the border on many occasions and to an extent it's half a dozen of one, 6 of the other. By the time you've driven 600 odd miles from the channel ports its very tempting to stop around Narbonne, plenty of pleasant sites around, rather than do the extra few hours on to the Costa Brava. Price wise I don't recall there being much in it, sites in high season are ridiculously expensive anywhere on the med. Fuel is indeed a little cheaper in Spain, as is eating out. To be honest though if you added up the total holiday cost (ie sites, ferry, petrol and tolls from the UK, shopping, eating out) I suspect that there wouldn't be a massive difference in cost either way, so I'd be inclined to base the decision on which area you like best. Personally other than a few exceptions I find the French resorts in the area a bit bland and prefer the Costa Brava, but no doubt others feel the opposite.