It is both ways, however there seemed to be a lower rate when we went there in October. We only paid 13.90 on the way there (Sunday afternoon at 4pm) but 19.90 on the way home on a Friday night.
Agree with Minnie Mouse - far easier by card! Saves having to have the cash! For the record paid 18.30 each way in December from Calais but that was using the A16 / E402 i.e. via Boulogne.
How much time does it add/ how easy is it, to travel off autoroute for some/all of the way? Ferry doesn't leave until 9.30pm the night we leave so could take a leisurely drive (unless we want to watch a parade once again in the afternoon )