That healthy foods cost more has become conventional wisdom, but a new study is the most thorough yet in calculating how much more: about a dollar and a half.

‘Before now, we've seen studies looking at prices of one or a few foods or diets, in one city and from one store,’ said Mayuree Rao. ‘And the results have been mixed, with some studies finding that the healthier options cost more and some studies finding they don't.’

Rao is a junior research fellow at the Harvard School of Public Health and a medical student at the Warren Alpert Medical School of Brown University in Providence, Rhode Island. She led the study that was published in BMJ Open.

The research team identified 27 previous studies from 10 countries that met their inclusion criteria and reviewed each of them. Fourteen studies were conducted in the U.S, two in Canada, six in Europe and five in other countries including South Africa, New Zealand, Japan and Brazil.

Twelve of the studies were market surveys that evaluated the prices of anywhere from two to 133 foods each and included up to 1,230 stores.

Fifteen studies were dietary surveys that ranged from 30 participants to more than 78,000.

The researchers compared the costs of the healthiest eating patterns with the least healthful and found that the healthiest diets cost on average $1.47 more per day based on actual food intake, or about $1.54 more per day for every 2,000 calories consumed.

The studies in the review used various definitions of ‘healthy’ - including comparing the amount of fat or sugar in similar products, or comparing whole grain versus refined carbohydrate versions, or looking at total fruit and vegetable intake or overall calories.

But all the findings were consistent across current standards for healthy eating, such as the Mediterranean diet, or Harvard's Alternative Healthy Eating Index.

The researchers also compared price differences in food groups - healthier meats and proteins had the largest difference between healthy and unhealthy choices - about 29 cents more per serving.

It's important to consider what an extra $1.50 per day can mean for individual as well as family budgets, according to Rao.

‘It translates to about $550 more per year for one person, and that could be a real barrier to healthier eating. We need better policies to help offset these costs,’ she said.

‘On the other hand, $1.50 is about the price of a cup of coffee - just a drop in the bucket when you consider the billions of dollars spent every year on diet-related chronic diseases like obesity, diabetes, and heart disease. When you look at the long-term health impact, the extra $1.50 is a good investment,’ she said.

Rao says that determining why healthier diets are more expensive is certainly an interesting topic for more investigation.

‘Other research from our group has observed that over the past century, the U.S. has developed a complex system of farming, storage, transportation, processing, manufacturing, and marketing that favors a lower cost of highly processed foods,’ Rao said. ‘We just don't have the same system to support healthier foods like fruits and vegetables.’

That extra daily cost can be a burden for low-income families said Adam Drewnowski, director of the Center for Public Health Nutrition at the University of Washington's School of Public Health. He was not involved in the new research, but some of his work was included in the review.

‘An additional $1.50 represents a 15-25 percent increase for the average American,’ Drewnowski said. ‘It does not sound like much but low-income families spend about $6 on food. So here, $1.50 represents a 25 percent increase.’

‘Also remember that $1.50 per person per day represents $540 per year, or $2,200 for a family of four. When you multiply by 200 million American adults (I am being conservative here), you get a total cost of 108 billion dollars - more than the entire USDA budget for food assistance,’ he told Reuters Health in an email.

Drewnowski points out that dollar figure is about the same as the estimated cost of obesity to society, said to be on the order of $100 billion per year.

‘So - are we asking consumers to spend another 108 billion in order to eat healthier? I wish they would, but I am not optimistic. At the very least we need a recognition that nutrition education needs to be accompanied by some economic measures,’ he said.

Drewnowski thinks the main problem is that empty calories have become extremely cheap.

‘Sugar, refined cereals and vegetable oils have made the food supply relatively inexpensive. However, those foods provide calories and (sometimes) few nutrients - so that obesity and hidden hunger can coexist,’ he said.

‘Subsidizing healthy foods and taxing unhealthy foods are evidence-based ways to address the price imbalance and nudge people towards a healthier diet. These are strategies that policymakers in many countries should be looking at,’ Rao said.