I drove to Dénia, Spain August last year.
After crossing the tunnel to give you an idea which way I took:
* Into FR: Rouen/Le Mans/Tours/Bordeaux/Bayonne
* Into ES: N121A/Pamplona/Zaragoza/Valencia
Observations: this route was quite boring. Very flat, from sometime after Bordeaux I just remember seeing an endless straight road and felt there was really not much to see until approaching the border and into ES and the terrain changed and became more interesting.
On the way back however:
* From ES: Valencia/Barcelona
* Into FR: Perpignan/Clermont-Ferrand/Orleans/Paris/Amiens/Boulogne-sur-Mer
Observations: this route was much more interesting coming back. Really bendy autoroutes going up and down the Massif-central mountains in FR. Was really interesting experience. I enjoyed this way.
Costs are return:
Crossing: £265 (notional as used clubcard points)
Fuel: €329.28
Insurance (breakdown-EU) £41
Tolls-FR: £155.74 (in GBP as used toll-tag, used a friends not activated for ES)
Tolls-ES: €67.95
Emissions sticker (for Paris mainly): £3.76 inc shipping
Further observations: I spread it out over 2 stops getting down there (near Tours and near Pamplona). The car did not have air-con, so had windows down, unrefined, noisy. I was drained when I finally got there. If I had a more refined car w/ air-con obviously would have been more comfortable, etc.
Would I do it again? Yeah, I do like a good road trip!