I often make soup, it's quick and easy and cheap. I also always keep a couple of tins of soup in the cupboard, usually Baxters which is around £2 a tin.
When I regularly cooked a Sunday roast I'd make soup with the leftovers, my kids used to refer to it as stoup, half way between soup and stew.
In the interests of economy I've made a pan of soup this morning, enough to last 2 or 3 days. It was the first recipe I learned on a vegetarian cookery course I went on about 45 years ago and I still make it.
I have no idea how much it costs to make but it is cheap, a packet of broth mix is £2 for 500 grams, I use a very small handful. I soaked the broth mix overnight and changed the water twice.
I used one potato, one carrot both approximately the same size, one onion slightly smaller, all these were chopped into smallish dice. You can add herbs or spices to your taste, I usually add pepper and curry powder. Bring to the boil then turn down to a simmer until everything is cooked through.
I brought mine to the boil whilst I was making my morning porridge and then turned the ring off. When I went back to the pan after breakfast everything was cooked, once it's cooled I'll transfer it to plastic boxes and store it in the fridge.