i have been every year since 06 and hoping to go again this year
if you are camping a abig group, if you plan you can eat and get very pissed everyday for 20-30 euro's a day
ferries approx 60 return dover to calais if you travel early and leave late, we do weds-mon
its abou 200km this side and 400km in france (from london)
do it, its the best trip ever, the cars, the atmos, the campsites are surreal at times, £30 tent and £150k supecar not uncommon!
also, dont forget to build a beer mountain....but you wont beat ours