According to the Environmental Working Group's report Good Food on a Tight Budget, these five foods offer some of the most nutrition for your buck -- each is packed with healthy compounds and costs less then 30 cents per serving!
To get even more for your buck, use those greens to make carrot top pesto (something I am always meaning to try, but never seem to get around to! Though I do put the tops in the juicer so they don't go entirely to waste.)