If you grab some dry red lentils you can add them into the mince (they take 15-20mins to cook so you can pre cook or mix with a stock cube water and mince into savoury mince style business). Will help the mince go further while keeping the protein up.
Then red lentils are the go - no pre-soaking and they boil up to nothing, just add thickness. Great in soup or any kind of stew. This is the cottage pie filling I made on the weekend:
onion & garlic fried in oil
fry 500g mince (added some cornflour so it thickens but optional)
added some tomato paste (or tin of toms, or some passata - whatever is cheap and tomatoey)
added 1/2 cup(ish) red lentils
added some stock power (or gravox)
added a cup of water
Boiled it up until thick - if you have frozen veg chuck it in at the end. Omit the cornflour and/or up the tomatoes for more of a bolognaise.
That and some mash taters on top - I got about 4-5 good meals