Take about half the clothes you think you'll need if there is a washing machine where you're staying. Take your own small pack washing tablets and some clothes pegs.
Definitely take your own sharp knife (you can buy a corkscrew if necessary).
Food is expensive in France, so I was glad I'd taken cupboard items from home (tuna, sauces etc). We also brought some frozen meat which was well packed with ice packs and stayed frozen until we arrived. Meat seemed very expensive there. Take your own bags to pack shopping in.
Picnic lots - that keeps costs down. We also BBQ'd quite a bit. Creperies/pizzeria/brasseries were reasonable - about the same as home, so we also ate out a few times. Restaurants are happy to serve you a pichet d'eau (tap water) which saves buying endless drinks.
Best petrol price we could get (we got home yesterday) was E1.36/lr. If you fill up at Calais (E1.46/lr) or at home (£1.10/lr), it will be cheaper than the motorway (E1.56/lr).
I took Euros (and a few travellers cheques in case we ran out) rather than using my card as it costs min £1.75 each time the card is used. Also, with the cash, it was easier to budget how much we could afford to spend each day without getting a bank statement shock on our return!
There seem to be lots of local fete/festivals going on at the moment and they're a cheap and entertaining day out. The one we went to had loads of free things going on the children could get involved in and have a go at.
The other thing we found is that the tourist info places are keen to tell you about the expensive things to do, but not always volunteering info about things the locals do cheaply or free (forest walks with decent play areas etc). Outdoor pools are usually cheap too if there's one near you.
I hope you get better weather than we did! Enjoy........we did (despite the unpredictable weather).