This is the trip we done last September:
Took 2 weeks and the total cost including:
Ferry
Fuel
Campsites
Tolls
Food & Drink
+ Spending money
Was €1700 for 2.
We could have saved about €500 by staying in cheaper campsites and avoiding toll roads but since it was our first trip we stayed in 5* sites (We're used to our swimming pools LOL
Also concidering we had to pay €500 for the ferry, it would work out a lot cheaper going from the UK.