Try getting your daily 2000 kcals from carrots, lettuce, spinach and water. Compare that cost to getting a few items from McDonald's dollar menu.
I'll do the math for you: One pound of carrots has roughly 190 calories. So you'd need to eat at least 10 pounds of carrots to get average your daily energy requirement. How much is a pound of carrots? $0.99?
A McDonald's Bacon Cheddar McChicken has 480 calories for $1. So you'd only have to buy four of them.
Use potatoes. 10lb bags are like 3.99. Rice, beans, corn, what'd do you think people in third world countries survive on? My grandma used it spent Saturday morning making ten loaves of bread for the week, very cheap to do.
Carrots, lettuce, spinach and water are ridiculously cheap.
If it's over-priced, branded, luxury convenience product in tiny quantities, then it's obviously going to cost more.
The other cheap way to eat: freegan.