Anyone who has watched 'The men who made us thin' and 'The men who made us fat' on BBC2 (see i-player) will know that junk food (highly processed food) is cheaper than freshly prepared food with good ingredients. Highly processed food is full with stuff which makes your body crave more and more, makes you ultimately fat, possibly ill, but is dirt cheap.
The most interesting thing is that diets do not work and cannot work long term as the diet industry people more or less acknowledged. Exercise doesn't work either in so far that it won't keep you slim if you eat junk in the long term. However, fat people tend to exercise less, which makes it worse.
All quite shocking and watch the series and I am pretty sure you will be a lot more careful what you eat. The program maker compares the food industry with the tobacco industry as they knowingly engage in very unsavoury practices in the hunt for higher and higher profits whatever the cost to the individual.