I eat very little starchy carb, which pushes up my food bill, as I can't eat cheap stuff like oats, potatoes, rice, bread, flour and pasta.
My basic formula is protein + healthier fats + non starchy veg. I limit my food bill to less than £120 a month, so that's £4 a day. I batch cook, look out for bargain veg, eat lots of eggs and have a good variety of foods with adequate mineral/vitamin intake.
I don't cost individual meals because, more often than not, they include left overs from previous ingredient lists and store cupboard items (herbs, spices, mustard, tomato puree, etc), but I doubt very much whether any meal costs £6, unless i decide to splash out on a steak or a particular sort of fish.