Cottage pie on a budget, I made one last week as we're a bit poor at the moment... get a tin of minced beef+onion (about 99p) and then use any tinned or frozen veg you have in your cupboard/freezer. I used half tins of carrots, peas and green beans. Make a couple of cubes of stock, about a mug full and pour it over the top and mush it round a bit (it's ok if it's runny, it will condense). Make some mash potato, take the meat mix out, put the mash on top and put it back in the oven for enough time to "brown" the potato so it looks crispy.
I made 4 dinners out of that! Just freeze whatever is left over.
Also, pasta and anything. Get passata/tinned toms and add basil (or just get a tomato and basil sauce from the shop) and chop up some spam or whatever processed meat you probably have a tin of somewhere in your cupboard (or use chicken sandwich strips/prawns/bacon bits/whatever meat),mix it into the sauce and pour on top of the pasta. Even with spam it tastes fine
You could also make carbonara the same as above - just get a carbonara sauce from the shops, stick any meat in it you want, maybe some mushrooms or whatever, then pour on top of the pasta.
Corned beef Hash - Tin or 2 of corned beef, mash potato, "sweated" onions. Stick it all in a big frying pan until the corned beef looks brown/cooked.
As the others said, bulk out some spag bol or chilli with more tomato sauce than meat.. try the butchers for big bags of cheaper mince than the supermarkets.
If I think of any more I'll post them. Thanks to you all too, I've been doing budget meals for the past week and you've given me some better ideas