One of the easiest ways to eat on the cheap is to eat breakfast.
42 oz. of Oats cost roughly $1.99 (sometimes much less - buy off brand, or from a bulk bin).
A serving is 1/2 cup of uncooked oats.
So, if you have oatmeal at least once a day, that's a meal for just a few cents.
Another option is polenta/grits. If you make it on the dry side, you can slice it and pan-fry, if you make it on the soft side, you can top it with margarine, cheese, veggies or stew. At less than $.10 per serving (pre-toppings) that is one heck of a cheap meal.
Then of course we come to beans. Beans for breakfast? Sure, why not?
You can make an entire pot of beans for less than $1.00.
Breakfast burritos, chili scrambles (tofu, or eggs if you're so inclined) and ful medames are all great options.
Foods traditionally associated with breakfast make great lunch and dinner options. Don't knock 'em, just because you think of them as breakfast.