I had a lot of picnics... in the park, in the hostel, at the Eiffel Tower - Generally, if you grocery shop for locally produced goods, prices are pretty reasonable - and options are plentiful. You can get a big ol' baguette at any boulangerie for 50-75 cents. A package of nice cheese (i.e. camembert) runs around 2e. A small package of ham around 1.50. All that can make for several sandwiches, and a decent bottle of wine to wash it all down is going to be cheap as well! Dropping 10e on a good meal once in a while is worth it - just stay away from the tourist traps that have English signage/menus and you can get a memorable meal for a decent price. Shop around. The locals have to eat, too, and with the crazy rents they pay, there have to be lots of non-tourist places with good prices.
Best deal/meal I found was this little bistro in a suburb called Clichy (my work's HQ happens to be there) and a full meal w/ coffee and dessert came out to 7.75. In fact, you'll find this in a lot of places around lunchtime, as many French workers are given lunch vouchers worth about 7e - so restaurant owners do well to keep their prices around the range of the vouchers.