Make spicy casseroles with chickpeas and carrots, tomatoes, harissa paste or chilli paste and a tiny bit of bacon or chorizo or chicken, but it's still nice without the meat.
Corned beef hash is cheap and lovely and filling.
Tinned sardines with spaghetti, some chilli flakes, a couple of ripe tomatoes, some dried herbs
Nigella's spagetti with Marmite and parmesan it's delish, but only cheap if you already have some Marmite!
Special fried rice/Nasi goreng or Singapore noodles with whatever leftover bits of veg and meat you might have lurking, and an egg broken into it.
Sausages or sausagemeat with braised lentils in chicken stock with herbs, bit of tomato puree, or a bit of crispy fried bacon for flavour, and some crusty bread.
Cheap ham hock or gammon joint, cooked really well so it shreds like pulled pork. It's very flavoursome so a little bit goes a really long way.
Use some to make wraps and quesadillas, or add to peppers and onions etc to make a stuffing for baked potaotes, or the fried rice or singapore noodles mentioned above.
Make some split pea soup, REALLY EASY AND CHEAP and yummy, - just boil the split peas in chicken stock, maybe a chopped onion if you have it, then blend it and add in some of the shredded ham hock, or just some bacon offcut pieces cooked until crispy.
When you buy jars and packets of sauces or tinned stuff if you don't use all of it straight away bung the rest in the freezer so it won't go to waste. I do this all the time with things like pizza topping sauce, jars of pesto, and fresh stuff like spring onions or ginger. I chop it or grate it or whatever and freeze it into nuggets or small batches so I only ever take out what I need. I hate wasting food.
If you are really on the bones of your arse but can stretch to some milk and a lemon, and a couple of eggs, dig out the flour and have pancake night. Or have savoury pancakes with bits of leftover cheese, and any bits of onion, mushroom or whatever you have.