What this new study did was acknowledge that healthful foods will keep a person satiated for a longer time than low-nutrition, high-sugar junk foods. A $1 apple will satisfy more than the $1 bag of chips. Once you account for that reality, junk food is not as cost effective. The same comparison can be done for oats vs. a doughnut for breakfast.
This knowledge led them to measure food costs in two new ways. One is price per edible weight, the weight after it's cooked and inedible parts removed. The other way was price per average amount eaten. I know I can only eat 1/2 cup of oats at the most, but can eat 1/2 dozen Krispy Kremes. I can eat an entire bag of Doritos, but can't eat the same $$ cost worth of broccoli. Even if I did, I'd be very full after the veggies and still able to eat after the Doritos.