The difference between the healthiest and the least healthiest food is $1.5 per person, a survey found.
Researchers carried out an overview of 27 previously published investigations into dietary patterns that had solid data about what people ate each day and how much it cost.
The studies were carried out in eight advanced economies and two middle-income countries, Brazil and South Africa.
Diets were rated according to healthiness: eating more fruit, vegetables, fish and nuts ranked higher than consuming more processed foods, meats and refined grains.
On average, a day's worth of the healthiest diet cost about $1.50 more than the least healthiest, the analysis found.
"This research provides the most complete picture to date on true cost differences of healthy diets," said Dariush Mozaffarian of Harvard Medical School.
"While healthier diets did cost more, the difference was smaller than many people might have expected."
Compared to the economic costs of diet-related conditions such as obesity, diabetes and heart disease, which have gained epidemic proportions in some countries, the difference is "very small," said Mozaffarian.
The gap between unhealthy and healthy eating is mainly explained by high-efficiency, high-profit systems to farm, make and sell processed foods, according to the paper.
This "inexpensive, high volume" approach could conceivably be applied to healthier foods, too, to bring down prices, it suggested.
"Over the course of a year, $1.50 per day more for eating a healthy diet would increase food costs for one person by about $550 per year," said Mozaffarian.
"This would represent a real burden for some families, and we need policies to help offset these costs."
Of the 27 studies used in the review, 14 were conducted in the United States, six in Europe (France, the Netherlands, Spain and Sweden), two each in Canada and New Zealand and one each in Brazil, Japan and South Africa.
The paper is published online in BMJ Open, part of the British Medical Journal stable of publications.