There's a myth going around that eating healthy is expensive. People use it as an explanation for why obesity is such a huge problem in this country. This idea, wherever it came from, is a pretty dangerous one. I've worked with both children and parents who think eating healthy is not worth it because it's beyond their financial means. As a result, they don't make the effort to pick out meals that are good for them.
While I agree that organic produce is pricey and your average person can't afford to shop at Whole Foods, that narrow subset of food choices is not the only type of food that fits the definition of "healthy." Healthy foods are simple, unprocessed foods, can be found at any supermarket, and provide a multitude of nutrients that keep you full longer. And, as Mark Bittman explains using the graphic below, healthy foods are often cheaper. By a lot.
Once you realize that choosing healthy foods won't wreak havoc on your budget, the next step is to learn to cook them.