Soya & beans
I'm surprised no-one has mentioned using soya mince. Holland & Barrett (and Tesco) do a 375g pack for £1.89 which when reconstituted becomes the equivalent of 1.5kg of mince. I use it all the time to make chilli, bolognese sauce, cottage pie, anything that has liquid and flavour and normally uses mince. The secret with soya mince is to add a little Marmite and a few drops of olive oil to the boiling water used to reconstitute it.
Dried beans are also a good buy, once they have been boiled and simmered, they double in weight. I buy the 500g packs from Tesco of red kidney beans, cannelli beans, pinto beans and black eyed beans, mix them together and store them in an airtight plastic tub. 2kg of mixed beans (1 pack of each) comes to £4.27.
375g of soya mince, 2kg mixed beans, 4 tins chopped tomatoes (Tesco Value, £0.31 each), 1 tube tomato puree (£0.35), 1kg onions (£0.90), 1/2 jar crushed chillis (£0.95/jar), 1/2 jar hot chilli powder (£0.95/jar), 1/2 bottle hot pepper sauce (£1.00/bottle), a couple of oxo cubes, 2 teaspoons Marmite, a little olive oil and some water gives me about 10kg of chilli for about £11.00. That's enough for about 30-40 people at a party or 20 large portions of chilli for my freezer, giving me 20 meals (rice or baked potato with the chilli would of course be extra).
Flame sign because my chilli should normally carry a chemical warning, I like it hot.