Well, I think it's actually more that unhealthy eating is inexpensive, accessible and tempting. Chips, crackers, white bread, stuff that is full of preservatives, high fructose corn syrup, saturated fats and white flour can be made inexpensively, and sold inexpensively.
But in reality, canned beans, fruit, vegetables; there are a lot of healthy items that are cheap, you just have to be more creative and put more time into cooking them.
In my experience, it is way more expensive to try to eat healthy as in organic produce, grain-fed meats and eggs, and antibiotic-free milk. I try to alternate what organic items I buy each month, I figure at least I'm giving everyone a little break from pesticides that way.