I think it depends on the county you live in. I live in Nigeria and fruits and vegetables aren't really expensive. The thing also is fast food isn't cheap in Nigeria. We have a Johnny Rockets and a meal is like N5,100 ($15) . Not many people can afford that much for 1 meal, whenever I go there there aren't more than 2 people there. A salad is like N1,700 ($5). The not so healthy food is literally 3 times more expensive than a salad.
By way of visual amounts, you can get more unhealthy food with less money. loke looking at the physical amount, and spending the same amount of money you will see that you get more unhealthy food. But, after consuming food you get full faster with healthy food. So, in the end you spend the same amount and eat less but feel full quicker and longer using less food and fueling your body with good nutrients for long lasting energy and all around better health.
Healthy food can be cheaper only if your willing to spend the time preparing it youself. Every Sunday evening I spend 1/2 hour cutting up lettuce, peppers and celery for my weekly luncheon of salad. $10/food for lunch vs. spending $40 week for prepared cafeteria food.