In high season it certainly can. There's a shortage of overnight stops around Calais, so basically it's a 'seller's market' and they can charge high amounts for an overnight stay. There are lots of campsites in France which charge a similar amount in high season - but you usually benefit from the other facilities available such as amazing pool complexes, bar, restaurant, indoor pool, sauna, etc - where with an overnight stop you don't tend to use any of these things.
There are several smaller sites - Les Epinettes at Peuplingues is recommended on other forums that I contribute to. Do you belong to the Caravan Club - there are others listed in there which are cheaper?