You need a balance diet: beans, rice and potatoes are good for carbs/energy, but lacking in lipid, protein and vitamin. For a cheap meal, the majority of it should be carbs, but adding 1-2 egg improve it a lot.

Many Asian cuisines (that have fed ppl who have been very poor) feature a dish that is a beaten raw egg mixed through rice (the hot rice then cooks the egg a little). Rice & beans etc are good staples but the experience of extremely poor people that is reflected in these cuisines is that it's good and worthwhile to add some extra good quality ingredients to these staples.