I hope this reply is not too late for you,
I drove to the South of Spain quite recently and it cost me about £300 including everything.
I have a Wife and a three year old, and it worked out cheaper than flying, and with no luggage restrictions!
My advice is Don't take the Toll roads unless you are on a strict timetable, plan on taking at least a couple of days to get there, and try to see something in France on the way.