If you fill your oven with several items and cook them all at once, rather than having your oven on for one pie, or fill a slow cooker to completely rather than half ful, you save on the cost of the fuel.
If I cook a casserole on a low oven, I always fill the pan almost full, to get 2 or 3 meals out of the one lot of fuel. A
If you have just a bit left over, and put it in the freezer, and repeat that several times, every so often you can pull all the oddments out, and have a 'free' meal, each person having something slightly different.
Also, hyou can ofetn get ingredients on bogof or similar, so bulk buy, bulk cok, and you will save money. much better to cok and freeze, than throw out the one you got free because you forgot to use it and it has gone out of date!