You're comparing a rather strange selection of foods. The cheapest foods per-calorie are the things you see the poor of the word eating: rice, corn meal, beans, etc. You can buy a massive bag of rice for almost nothing, and some countries subsidize that further. When people talk about the poor living on a dollar a day, they're serious.

You may notice that the poor eating those foods, and not dramatically more expensive items like soda, aren't getting fat.