How about soups? Soups are easy to make, can be made with whatever veg and/or meat is on offer at the supermarket, and if you blend them you can hide all manner of things he thinks he doesn't like in them (like rice, for example). Carrot and coriander soup costs next to nothing to make.
How about fish that don't come in finger form? You can get 6 frozen coley steaks for £2.79 in Sainsburys (I checked) and that'll do you 3 meals. You can spruce them up by making your own sauce out of some milk, cornflour and parsley. Or add some mashed potato and you've got a fish pie.
You could grow your own herbs on the windowsill.
Get chicken thighs instead of chicken breasts, much cheaper.
If he likes sausages, do something interesting with them. How about a sausage and lentil casserole? (
Recipe here). Or squeeze the meat out of the sausage skin and make meatballs.
You could try him on things like pearl barley instead of rice, or maybe couscous or quinoa. All those things are much cheaper to buy if you buy them in your local Asian food shop.
Do some interesting things with veg - instead of boiling carrots, roast them with some honey drizzled on. Instead of potato mash, make it with parsnip or swede. Or do a different kind of potato dish - pommes boulangere, for example (
Recipe). Doesn't cost any more than mash would.
Cheap traditional food doesn't have to be boring.