This can actually be formulated as an interesting (constrainted) optimization problem.

Given

* an amount of calories

* a distribution of macronutritions (protein,fat,simple carbs, complex carbs)

* a certain variablility (at least 20(?) different foods)

do minimize:

- Cost

Interesting problem.

A more thorough database with micronutritions would even yield a healthier result

Edit: Look like it's not new: http://en.wikipedia.org/wiki/Stigler_diet