Paul
I knew there was some previous info on this subject
http://www.eyeonspain.com/community-forum/24311/uk-to-roda-by-car.aspx
viamichelin is certainly good starting point should help with tolls and cost of fuel, shortest mileage from Dover is 1200 miles, what is your cars mpg? How far do you want to travel each day as that is 24 hours of driving in total.
Quickest route is round Paris and head down to Orleans, Clermont Ferrand, Millau, you must stop at the bridge, Perpignan, then into Spain all the way down the coast BUT there are altrnatives such as going to Rouen then West of Paris and west side of France and over,under or round Pyrenees, more expensive on tolls
How many overnight stops do you want, is it a quick trip or a sight seeing opportunity?
Cost depends so much on stops, mpg and tolls etc but approx £500 each way would cover it working out much the same as doing the ferry route and if you are using the car for a while i.e. a month it saves on car hire.
Happy to try and answer any specifics