Lentil shepherd's pie is very cheap - lentils are reasonably priced in all supermarkets. Cook in stock along with onion, garlic, tomato purée, peas and grated carrot. Put mashed pot/sweet pot/turnip on top and cook in oven. Yummy! 50g of lentils per person. We usually make for four and use the following day.
Anything with pasta which is relatively cheap, you can get cheap tinned toms. Or I love Aldi baked beans I think they are 29p a tin so lots can be made with them, such as cowboy pie, I use them instead of kidney beans in chilli, or eggs, beans & potaotes.
Use your store cupboard items up & anything in the freezer. Try bulking out meals with tinned toms.