Well, it’ll vary somewhat depending on your location, but it’s generally going to be some combination of grains and legumes for your protein and most of your Calories, some veggies, mostly brassicas, for vitamins, and a bit of whatever the cheapest fat is in your region. You might also add small amounts of spices to make it more palatable: Spices are expensive by weight, but you need such small quantities that they end up being very cheap on a per-serving basis.
Partly, too, it’ll depend on what you mean by “cheap”. For instance, there are a lot of places that give away day-old bread for free: Does that count as cheap, or do you count it by the price the bread would have sold for (it’s usually pretty fancy bread)? What about home-grown garden vegetables? You might spend very little money on them, but they need land and time.