Fresh produce is extremely cheap, if we’re talking about the same thing - how much is it for you to buy 2kg of potatoes, carrots and onions from the supermarket, and how much food can you make from that amount of veggies?

Edit: Also, don’t go out of your way to avoid fat in the sauce, but avoiding dairy is a good idea. Using olive oil as part of a sauce for example, is perfectly healthy. Fats are good for slow release energy.