Most Helpful Guy
Depends on how cheap you're looking for. I've been desperately poor before, so I can share that end of the spectrum. At that point, I broke food down into 3 categories: Calories, vitamins and protein.
For calories, the cheapest way to get them is rice, hands down.
Vitamins are easy. You need some fruits and veggies and most of these are pretty cheap, so get something you like.
For protein, the cheapest way to get it is beans. I personally prefer black beans, so I went with that.
Finally, there is an art to making this palatable after you've eaten it for a week straight, so creativity and few bucks on spices can go a long ways. Probably my best creation was "Spanish rice" Rice, black beans, a can of diced tomatoes and a taco seasoning packet. It tasted alright, fed me for a couple days, and cost about $5.