Is it really? I thought adding more carbs to your diet (e.g. an extra bowl of rice or a side of mashed potatoes) is a really inexpensive way to stuff oneself, and a pretty healthy one too. Rice, buckwheat and all sort of cereals - they do you good. And here is a trick for cheap fruits: perhaps you have some nearby farms where you can pick produce yourself and then buy what you picked for a much lower price than in the store (and it tastes better too!).

And there is always peanut butter!!! That's a good measure of calories for your buck.