Hi Carl (or Thelma65!)
It probably is all in the timing. Just as a 'test' I checked the Alan Rogers/Camping Cheque website which I mentioned above, and using the details of a 5 metre car, caravan between 6 and 7 metres, two adults, going out on the 7th August from Portsmouth to Caen at 14.45 and coming back on the 21st from Caen to Portsmouth at 8.30, the quote came in at £514.85 (that's including 3 Camping Cheques worth £42.00).
Trying dates of outward 21st August, and back on 5th September (the Caravans go Free offer is available after the 21st August), the quote is £312.35 including the Camping Cheques, again worth £42.00 - saving more than £200!
There are many of us on here who holiday, now, outside peak season, knowing that you can make real savings - not just on ferry crossings, but also on campsite fees too. Even those with children can often squeeze two weeks in at the end of August/beginning of September to benefit from the lower prices at that time. The camping/caravanning season in France is very short - and most of the French have gone home by the 15th August, so after that many sites start to reduce their prices, or accept an ACSI card or Camping Cheques or similar, so you can also benefit from these. If you use an ACSI or Camping Cheques sites then they also often do special offers such as 14 nights for 11 Camping Cheques - which brings the price down to around £11.00 per night, which includes pitch fee for car and caravan (or tent or motorhome), an awning, two adults, and electrici hook-up - and on a few sites even a private bathroom on your pitch if there are any available!
There are also lots of other smaller sites where you can get a pitch at a reasonable price - depending on the area you're heading for.