As everyone has said, your fixed costs will be the ferry, and fuel. You can cut down the ferry cost by using one of the shorter crossings, and the Caravan Club are 'hinting' that they may be making some 'very special' offers in the New Year. Fuel you can't do anything much about if you want to drive to France. On sites, however, if you can go for the last two weeks in August, rather than late July to early August then you may find sites are slightly cheaper, and even possibly be able to use one of the discount schemes such as Camping Cheques or the ACSI card. *see
www.campingcheque.co.uk and
www.campingcard.co.uk. Each offers a discount over the 'normal' site tariff, usually in what is considered low season, but that can be any dates, and some sites accept them all season. ACSI card offers sites for 10, 12, 14 or 16 euros per night, and that includes a pitch, two adults, car and caravan, awning, and electricity. Camping Cheque sites cost £13.95 per night for the same 'package'. The 2013 ACSI Card dates are now available on line, but Camping Cheque haven't yet announced their dates for 2013, so it's hard to see which sites might just be available.
The Vendee isn't an area that we've visited - we prefer the more wilder parts of France. Just as an example of what you might get at the back end of August, is Les Charmilles, in the Ardeche (which is much further south, with more dependable weather) which is an ACSI card site, which accepts the ACSI card from 17th August, and includes up to three children under 6, in the 16 euros per night price band, but does 14 nights for 12. That means you'd pay 192 euros for 14 nights, plus a supplement for your 8 year old of 4 euros per night
So your spending could be reduced to:
Ferry £78.00 return (DFDS offer)
Campsites 14 nights at around £250
Overnight campsites in each direction £40.00 or £80..00 if you want two overnight stops
Fuel £250
So a total of around £656.00 (or less given the exchange rate) - which is rather less than your total of £1500 - but a very different holiday!!
The Ardeche area is a wonderful area for holidays - with wild scenery, a great river, some fantastic wildlife - birds of prey, beaver in he rivers, fishing in the river, and beautiful mediaeval villages with lovely markets - and is where we spent many happy years when our children were small.