Large open cup, horse or flat mushrooms have a good taste. Although they may cost more than the button ones, you only need a small amount. Frozen vegetables are just as good as fresh and they may be cheaper. You can use them in cooking e.g., in a risotto or pasta dish or as they are. Things like frozen peas or sweetcorn are cheaper, but bags of frozen sliced green and red peppers or sliced mushrooms can be good value as you only need a little in a dish.
Tinned tomatoes have a hundred and one uses.
In a pasta sauce, as a pizza base, in shepherd's pie, in a soup, in a curry. See BBC Food or the NHS Healthy Living site for more ideas. The majority of people can't do without their meat and fish. The great British Sunday roast or burgers for the kids are what people expect. Try some of these money saving ideas.