My budget would be a bit more profligate than Brains. Say
- 10€ for campsites (but see the thread on camping costs)
- 3-5€ for a picnic lunch
- 5€ for a dinner (say a tin of ready-made stew) plus bread
Breakfast could be muesli/powder-milk mix (rehydrated) plus a cup of tea, but if like me you like to have a second breakfast allow about 3€ for coffee and croissant.
Beer IIRC a couple of euros a pop - cheaper if you buy from a supermarket, but, as Vernon says, a cold beer really hits the spot.
A while ago I did a post on prices of 'typical' food items in France if you really want to get detailed about it.
I'd allow a bit more for 'treats' - eg a hot meal in a cafe on a day when it's rainy and horrible.