Could you get away with bulking out a smaller portion of 5% mince with frozen quorn mince or lentils plus lots of SS veg? This could make spag Bol, chilli, shepherds pie, etc depending on herbs added. Eggs are good - as you said omelettes or a frittata with lots of SS veg added plus ham or bacon (you don't need very much to get the flavour). I make a large roast chicken do 2 or 3 meals - so start with a roast, then use leftovers to do risotto, curry, paprika chicken casserole, etc (lots of receipts on the SW website). You don't need much and can again bulk out with veg and other protein (beans or pulses). Good luck - hopefully someone else will be along with some other great ideas
In our house our cheap n cheerful meal is egg fried rice. I'm vegan and SW so have no eggs and loads a veg dp and dss have bacon bits cut up, mushrooms onions peas sweet corn all fried up with fry light and soya sauce. Very yummy and mega cheap n filling.
If I buy cheap mince ( I try not to but there were many times when I had to) I would fry it for a bit with no added oil (until the fat in it began to run) then put it into a colander and pour a kettle full of boiling water over it. This removes a lot of the fat content but also a lot of the flavour! Therefore I would put it back into the frying pan with an oxo crumbled in and continue with the recipe!
I still do this with lamb mince which I find way too greasy.
There are some gorgeous recipes on the Hello Fresh website, I have made the salmon and courgette linguine one several times and it is really cheap and filling as it only uses 2 salmon fillets (£2.30 ish in Lidl). Courgettes are often reduced in our supermarket.
The leftover chicken leg meat is nice if you shed/chop it and sprinkle on some fahjita spices then serve it in a wrap with loads of roasted veg.
I bulk out beef mince in bolognese etc with tons of speed veg, definitely makes it go further! We go to the local butcher too, where the meat's much cheaper.
Buying whole chickens instead of packs of breasts etc works out cheaper - can do it as a roast dinner, then have leftovers in sandwiches, salads and soups etc. Or tomato based pasta sauces with cheap tinned tomatoes and some seasoning - or soups made with loads of speed veg...