I've been pondering this question and wondering if in part is not actually always true. Then, I came across this article and am still not convinced by their "calories/per serving/price" logic. I guess it depends on what your definition of 'healthy food' is and also what you consider to be expensive. I think this is a common excuse people use for not changing their cooking, shopping and meal planning habits. They assume they can't afford anything healthier.
BUT....have you ever looked at a price of a bag of potato chips? Yikes! Those bags are expensive, considering what you get, not to mention the bag is half full of air! Around here they're usually over $3. So you can get a bunch of salt and other not so good for you things from them or you can buy a 3 lb bag of apples for about the same price.
Now, I get that certain times of the year produce prices do go up, but still, on average you can get a pound of fresh fruits and veggies for under $1, if you're shopping sales and buying things in season.
Not to mention dried beans, which are full of nutrients & protein (which will actually fill you and give you energy much longer then a candy bar will) and have to be one the cheapest things you can buy! Whole grain pastas really don't cost much more then the regular ones and you can find whole grain breads for reasonable prices too.
Like I've mentioned before you can buy natural meats in bulk from local farmers to cut costs and eat healthier as well as the fresh, hormone free milk and other dairy and egg options.
All I'm saying, I guess, is that there are options. If you want to have a diet high in fibers, proteins and fresh fruits and vegetables, you don't have to break the bank to get it. Now, if you want an all organic diet, you'll pay the price for it. But is that the only definition of "healthy"?
Maybe it's just a matter of starting small and switching out the french fries for a baked potato or the chips for a handful of carrots or apples.