One of my favourite cheapies on a green day is a baked bean shepherds pie. I add any herb or spice I fancy to the baked beans or not depending on how I feel. Put into an oven proof dish top with SW friendly mashed potatoes and bake in the oven until the top is crispy.
I also add a tin of baked beans to 250 grams of mince meat and use half to make SW chile and half with onions and carrots to make savoury minced beef dish.
Tinned tomatoes blended and with cumin added makes a super, cheap soup, forgive the really bad pun.
Buy peppers, red, green, yellow when they are cheap and roast them and serve with baby potatoes, no need for meat and so tasty.
Pasta and tinned tomato and if you have oregano or basil in throw that in with the tomatoes. Cheese if you have it cooked in with the tomato if not it still tastes good.
Eggs are cheap, and nutricious and free and versatile.
A proper Spanish omelette is made with just potato and onion. I boil the potato and onion with salt and pepper. Put them in the omelette pan, pour in the egg and cook as normal. A tip for turning the omelette as it should be thick, tall if you know what I mean is to hold a plate over the top of the pan turn the omelette onto the plate and the slide it back into the pan. This is delicious and sooo filling.
I could bore you with loads more but I am sure you get the idea.