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)

Pizza, esp with sausage on. It's basically just fat, and costs very little to buy.

A Big Mac is actually only 500 cals, a double Whopper 1100.

Pizza can end up being 6000.

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.

[1]: https://efficiencyiseverything.com/the-highest-calorie-per-d...