Cabbages, dried beans/peas/lentils, and collards. Learn to cook these "poor foods" in ways you like - they are cheap and plentiful and amenable to lots of different methods (cabbage, especially). Extend/bulk all the above with rice, farro, polenta, tortillas, or a solid southern cornbread recipe (not sweet and more cornmeal than flour).

I do both collards and beans in an instant pot (not together, though), which cuts down on mess and time. One of my favorite things to do with cabbage is make Kim chi or sauerkraut. Kraut is easier (and a little cheaper, it's just cabbage, salt, and time) and opens up a realm of easy Euro-ish meals (kraut, lentils/potatoes, and sausages, for example, is highly economical, tasty, and filling).