Pasta- carbonara is cheap , 250g pasta, two eggs, a 50p pack of bacon cubes, an onion, a handful of frozen peas, a handful of cheese.
Soup is always cheap- base it on any cheap veg- carrot, tomato and red lentil, leek and potato, parsnip, cerleriac( these always end up in the cheap aisle in asda- I think people are put off by their hideousness )and the tiger loaf for 50p to go with.
Or chunky soups- ribollata can be made with a tin of white beans, a tin of toms, an onion, a carrot , stock and a handful of kale/cabbage/cavolo Nero( again often in the discounted aisle)
Minestrone is equally cheap and delicious
Split pea soup
Vegetarian fayre is generally cheaper- you can make a lentil Bolognese for half the price of meat if not a third. Same goes for burgers- see the bean burger thread- a tin of butter beans is £0.40 in asda, haricot and kidney are cheap too ( though not cannellini no idea why) a pack of dried chickpeas is £1.19 but will make the equivalent of three family meals- a chickpea and lentil loaf( like meatloaf) a chickpea curry, and chickpea burgers.
A pumpkin can last too- they are usually about two quid but last for three meals if paired with pasta/ noodles/ pulses- chickpea and pumpkin stew, pumpkin lasagne, pumpkin risotto for example.
I am a huge advocate of meal planning when on a budget. If you can do it it will save money, but I know some don't like to. I find it reduces waste and saves money.