The economics may be influenced by where you are starting from. If Dover is nearer or even breakeven distance for you than Portsmouth, then the short crossing may well be cheaper, even allowing an extra night or two in France.
Also it depends on where in spain you are going. If to the south coast or as far up the east cost as, say, Mojacar, then the route down the west coast of France and across to Valencia via Zaragoza is a pretty good straightline and avoid the potential trouble spots of Madrid and Barcelona.
I would check out your dis;tance to Portsmouth or Calais and then check your overall distance to say Bordeaux or St Jean de Luz and see how the costs work out. While there are not all tham many French sites open all year there are enough if you plan carefully, similarly across the middle of Spain. If you get a CC Volume 1 Europe site guide you will be able to find such sites at Poitiers, Pons, Bordeaux, St Jean de Luz in France and at Catalayud, Manzanera, Navajas, Moncofa, Murcia in Spain.
A reasonable way of doing the check is to use www.viamichelin.com and put in the appropriate data - this shoudl give you overall costs.
For fuel assume France is about UK prices, spain about 10% cheaper. The french autoroute tolls can be very expensive, particularly south from Calais, whereas you can get south of Nates on the west coast route toll-free.
Have a good browse round on a wet Bank Holiday !