Bag of frozen Mediterranean veggies, about £1, box of tomatoes 50p asda, packet of pasta 50p asda. Cook the veggies in a pan or the oven, add the tomatoes with some garlic (maybe) bit of chilli sauce and mix in cooked pasta. If you're feeling flush then add some garlic bread.
Fry off any veg, onions, peppers etc with leftover Ham, bacon or chicken chopped up, bit of tomato purée, any spices (vinegar, sugar, salt and Chinese spice/Cajun/Italian/whatever) then add cooked rice, stir through and cook for five minutes, done!
I find buying frozen veg, especially chopped mixed peppers/sweet corn means you can throw a handful in almost anything. Bulks it out.
Pork shoulder steaks are cheap, add a couple to the slow cooker with some bbq sauce and a chilli. Cook low, shred, add more bbq sauce and a tin of mixed beans in water (50p). Serve on cheap tortillas with cheese, microwave for a minute. Or put into wraps with cheese inside and micro. Serve with wedges and coleslaw (homemade is amazing) and salad.