For DIY food my wild guess would be butter. My somewhat educated guess for prepared food would be something off the value menu at McDonalds/Burger King/Wendys.
My first guess would have been rice. Rice is super cheap. According to [1], it's oils and flours, followed closely by rice and sugar.
Butter is around $5/lb for 32 x 100-Kcal servings, so it's only around 640 Kcal/$. That would put it pretty far down the list, below milk and eggs, and on par with lentils.
I thought someone measured twinkies to be the best... That was a few years ago though, we may have found something by now that's even better. (or worse, depending how you look at it)
