First of all, even those articles are untrue unless your time is valued at zero.
Secondly, there are just as many - if not more - studies that show that junk food is, in fact, cheaper. Because weight is an obvious, but subtly wrong, measurement to compare by:
"Healthy eating really does cost more. That’s what University of Washington researchers found when they compared the prices of 370 foods sold at supermarkets in the Seattle area. Calorie for calorie, junk foods not only cost less than fruits and vegetables, but junk food prices also are less likely to rise as a result of inflation…"
http://well.blogs.nytimes.com/2007/12/05/a-high-price-for-he...