Grocery stores have discovered the perfect product, and customers are happily paying higher prices

The nutrition bar industry is booming, and grocery stores are thrilled.

"Stores love them because they are individually wrapped and have a long shelf life, yet they are popular and turn over fast," writes Alana Dizik at The Wall Street Journal.

The price of the average snack bar has doubled to $2 from a decade ago, but consumers are happy to pay steep prices for the bars, WSJ writes.

Stores also make big profits from the convenience food.

"The cost of ingredients makes up only 25% of the price, and profit margins for bars tend to hover as high as 40% to 50%, compared with only 20% to 30% for most other packaged foods," WSJ writes.

Consumers are willing to pay up for bars from brands like Kind and Quest because they believe the protein is a good value.

Manufacturers have taken note of the opportunity: there are 1,012 nutrition bars on the market today versus 226 a year ago.

The Kind brand faced a scandal after the Food and Drug Administration suggested the bars weren't healthy.

In a warning letter addressed to Kind, the FDA said at least four types of Kind bars were in violation of "healthy" labeling rules, Bloomberg reported.

The FDA claimed the bars have too much saturated fat to be labeled "healthy" and not enough protein to use the plus symbol.

The brand adjusted its labeling after the criticism.