Personal responsibility absolutely plays a role here.

I mean, I feel like the argument about food deserts and poverty and such has some validity. But otoh, I have known many overweight people in my life with good jobs and plenty of options and opportunities to buy real food, and they don't do it. And imma say that's their fault.

The cheapest food would be raw ingredients like rice, beans etc. That way you can eat an appropriate amount and not feel hungry. Also let's not pretend like frozen veggies are more expensive than McDonalds or other restaurants.

Otherwise eating less is always cheaper, you just might feel hungry.