There definitely is a social issue involved in getting healthy foods and it does depend where you live. In urban areas, there have been some studies that certain neighborhoods may not have grocery stores because the grocers don't want to be there. So in order to get groceries, the people of those neighborhoods either have to rely on convenience food or they have to travel outside of their neighborhood.
All things being equal though there are some certain issues in the US that make certain things cheap (corn products) and other things not so cheap. It is because corn is heavily subsidized in the US so everything from pop tarts to coke to beef industry benefits from that. It would be nice if all farming was subsidized so that it would create a more even playing field but certain industries lobby heavily to keep corn subsidized.
That being said, I do think that most people can find means to eat healthier on a limited budget but it definitely takes work.
You can't out-exercise poor eating habits.