If you think about each food group you need to shove into a meal to get it nutritious (sort of) and balanced, then root in the stuff you already have instead of going to the shop, you will save money. It's a (not very funny!) standing joke in my house that I nip out for bread and milk, and it costs £20 and comes in 3 bags by the time I've finished.
So - Carbohydrate foods.
potato, pasta, noodles, rice, bread, couscous, porridge oats, bulgar wheat.
Protein foods - meat, poultry, fish (tinned oily fish can be pretty cheap, as is tinned tuna), eggs, cheese, lentils (yuk, personally
), baked beans, seafood.
Vegetation - the obvious, but basic stuff like apples, pears, oranges, bananas, carrot, brocolli, cabbage and frozen veggies are cheaper than exotic stuff like stirfry mixes, mango, papaya etc. Salad also works out quite dear.
Fill your freezer with the proteins you want for the week, then just ad whatever carb and veg you want to it, whichever seems like a good idea at the time. You can bulk-buy pasta, noodles and stuff, and freeze bread, so if you have the space, you only need to go to the shop once a week. Especially if you can tolerate UHT milk