In reply to Noxeal:
I'd agree with McKEuan - around £300 for a week living a little frugally - bread and cheese morning noon and night. If I was going for a long weekend I'd probably stretch to using the tunnel (depending on where you live) as it does save a bit on driving but definitely saves on time so you can possibly make more of your time out there.
There has been a bit of a clamp down on camping in the forest so now I use the aire outside Milly if times are hard although a weekend in the back of a VW fox is probably going to make or break a friendship in equally bad ways but if you are on a tight budget you might get away with bivvying out... Musadiere is a nice campsite though and as others have said, is walking distance to a lot of good areas.
As an aside, check this out before you go - not read it fully but think it applies across France -
www.certifcat-air.gouv.fr
new rules on pollution levels for cars - I think you need a sticker to say how environmentally friendly your car is.
As another aside, the Liber-t tag direct debits your peage fees and reduces all the faff looking for Euros. www.saneftolling.co.uk. If you want a more accurate idea of fees, this site also has a link to the peage charges across france - autoroutes.fr.
Anyways, I hope this helps.
But also at least tell us
What colour your car is?
What tyre size you plan on travelling on?
Contiental or pirelli?
Do you skip leg day?
If not, how much leg work do you do? Take this number multiply it by X, divide this by the number by the radius of your tyre and then swap that number with the first number.
Gite or campsite?
How many pillows?
Down or synthetic?
Perhaps then we can help.
£300. Have a good trip.
Oh and don't speed either, it's really naughty.