Don't forget that you can get a 10% discount for booking early. use the code FRANCEC15, and if you go to France and return with Irish Ferries between start of June and end of August, you also qualify for a free trip to Wales in the off peak times (sept to end May with some exceptions)
If you can make use of this offer it will claw back about 400 euro for you. We made use of this last year and are planning are early trip to Wales next season.
My estimate for this years trip to France in June is about 500 euro. 6.9m length over 2.25 height 2 bed with window.
Davy