Assume that unhealthy food is $1/serving and 500kcal/serving. Assume that healthy food is $2/serving and 400kcal/serving. Assume that you need 2000kcal/day.

Assume that unhealthy food is $1/serving and 500kcal/serving. Assume that healthy food is $2/serving and 400kcal/serving. Assume that you need 2000kcal/day. A healthy diet costs $10 for "just enough calories". An unhealthy diet costs only $4 for the same calories. If you ate 3000kcal/day, it would still only cost $6. So yes, it can be cheaper to eat a lot of unhealthy food than to eat only a small amount of healthy food if the price differential is large enough. It's also easier to overdose on unhealthy food because the calories are so concentrated. A second bowl of ramen can contain 400 calories. A second head of lettuce might only have 40 calories. Of course, you might face a million-dollar loss 20 years later because of heart disease, but would it really enter the calculations of a single mom with a weekly grocery budget of $35?