Rice may be cheap, but it doesn't provide the protein, and almost everything else at the $2 limit is also going to lack the fiber and vitamins.

How about Feijoada? Mostly black beans and rice, which are both ridiculously cheap when bought in bulk. Years ago (don't know about currently) it was good enough to be the basic staple of Brazilian army conscripts. And one thing we know for sure is that soldiers burn a lot of calories.

Wikipedia claims[1]:

   Rice is rich in starch, an excellent source of
   energy. Rice also has iron, vitamin B and protein.
   Beans also contain a good amount of iron and an
   even greater amount of protein than rice. Together
   they make up a complete protein, which provides
   each of the amino acids the body cannot make for
   itself.

To supplement that, wouldn't cheap vitamin pills be good enough to keep you alive?

I know it doesn't sound appetizing, but "beggars can't be choosers". It sounds a lot more appealing to me than trying to live on ramen. And it sounds a lot healthier to me than most of what McDonald's sells.

[1] http://en.wikipedia.org/wiki/Rice_and_beans