All this talk about foods being fattening or not is besides the point. What it really comes down to is how much we eat. If you eat less kCal than you burn in a day, you lose weight. Eat as much as you burn and you stay the same. Eat more than you burn and you put weight on. You could (it would be very unhealthy) get away with eating McDonald's daily, as long as you ate less kCal than you burnt. Eating healthier food allows you to consume a higher volume of food before you reach the limit, same as exercise increases the limit. There are many apps to work out roughly how much you should be eating and that do all the tracking for you - MyFitnessPal being a great example.
I completely agree that healthy options should be cheaper, and that many people can only afford junk food, but there is nothing actually stopping them from only eating what they need, instead of what would be nice. It all comes down to self control.
Obviously there are exceptions to the rule, and this should not be considered advice for anyone - if you're concerned about your weight you really should consult with someone that has a degree.