1. Since most Americans still opt for the "unhealthy" foods these products are more likely to be produced en masse and come out to a cheaper price.
2. Most of these foods are loaded with all sorts of chemicals and flavor additives to the point where the quality of the meat isnt that important, and it comes out to a cheaper price.
3. Organically grown food is a much more expensive process, and organic farms are usually smaller than commercial farms.
I think it all comes down to relying on a few good staples, brown rice, cottage cheese, someone said tuna. Knowing how to properly marinate chicken. In the end I think eating healthy can cost the same or less than eating whatever you want, but it does take a certain amount of work in finding healthy cheap foods and preparing your own meals.
But it is ridiculous how an organic turkey sandwhich fresh cut at my school costs 8 bucks compared to the taco bell 4 bucks for a grilled stuft chicken burrito.