Well I should've mentioned that I live in Paris. Obviously if where you are you can get groceries for cheap then I'm sure you can easily beat the restaurants. Buying fruit, vegetables and meat/fish in small shops in Paris can be ridiculously expensive. Restaurants generally buy in bulk from vendors outside of Paris with much more reasonable prices.

Cooking would probably still end up being cheaper in most cases but not by a huge margin. But then again Paris might not be very typical in that regard, food is notoriously expensive here.