Right, I was wrong. The best calorie/$ ratio is with a bottle of cooking oil. (7-9k calories/$)

But you cannot eat that, so potato chips are a distant second (at least from the products I checked)

And no, the calories in chips added by the oil offset the lesser amount of weight you buy per unit - at least in my locality.

If I just look at calories per dollar, potato chips seem the most cost effective - at least as long as you take the cheapest ones

Pure potatoes are somewhere around 200-500 calories per dollar, cheap potato chips are around 1000/dollar

Pure noodles or rice might be even better, I didn't check those admittedly.

I just searched Google for "highest calorie tuber" to try and see what the best choice would be (cassava/tapioca/yucca) but Google was very insistent I see the search results for "highest calorie tube".