I have just come back from Bilbao, paid £302 but that of course is off season and one person. This was the Economie which meant the van was open deck and got covered in salt crystals. Would have been cheaper but was last minute booking so couldn't use the C&CC discount.
Personally I find it deadly boring being on a ferry for about thirty hours and wish I had driven back through France. Probably would have been quicker as well. Don't forget you also have the cost of meals on the ferry.
Shan't do it again. Will likely do Newhaven (which like Portsmouth is easier to get to than Dover from the north) on DFDS whose prices are very reasonable imo to Dieppe, Rouen (free), autoroute Le Mans, Tours, Poiters (tolled), N10 to Bordeaux (free and now nearly all dual carriageway) then down to Biarritz and the border (this used to be free but is now partly tolled)
You could do Portsmouth -Caen/Le Havre but his only saves a couple of hours and is more expensive (Brittany Ferries again)
The motorway over the border is tolled as far down as Bilbao, but then you have the choice of all the free motorways.
Use via Michelin to check costs.
Couple of maps here may help.
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Edit: just noticed WGs post. The few times we have used the BIlbao ferry it hasn't stopped in France. Just gone straight over. Very rough last year, return crossing was cancelled.