Reddit mentions of Rice and Beans

As an addition to the other great answers: I know of two cookbooks that are aimed at making food that is cheap and good (as in healthful and tasty). One is Good and Cheap by Leanne Brown, available for free from her website. The other is More-With-Less, which is published by a Mennonite group. I have never used Good and Cheap (it was in the news after it was published), and I've only used a few things from More-with-Less, so I can't really speak to their overall quality.

My personal favorite simple, cheap, tasty, and versatile dish is beans and rice. I typically combine equal amounts black beans and brown rice (I'm glossing over cooking them), season with cumin, garlic, and salt, add some diced tomatoes and lime juice, and top with fresh cilantro, cheese, and avocado (and hot sauce).
Using brown rice instead of white rice adds minerals and fiber, but isn't necessary. Avocados and cilantro are delicious, but aren't necessary (and can be expensive). Note that dried black beans require several hours of soaking before they can be cooked (and about an hour of cooking time). You can use things like canned beans, minute rice, canned tomatoes or salsa, and guacamole to make this basically effortless (though probably a little more expensive). There are lots of ways to combine beans and rice (there's even an academic book about it).

An even easier version, which I've only ever served as a side dish but is nutritionally acceptable as a main dish is: combine a can of beans with a can of corn (both drained), spice to taste (I use salt, cumin, garlic, etc), and add some fresh cilantro. Optionally add a drained can of diced tomatoes.