Most Americans consume diets that do not meet Federal dietary recommendations. A common explanation is that healthier foods are more expensive than less healthy foods. To investigate this assumption, the authors of this USDA study compare prices of healthy and less healthy foods using three different price metrics: the price of food energy ($/calorie), the price of edible weight ($/100 edible grams), and the price of an average portion ($/average portion). They also calculate the cost of meeting the recommendations for each food group. For all metrics except the price of food energy, the authors find that healthy foods cost less than less healthy foods (defined for this study as foods that are high in saturated fat, added sugar, and/or sodium, or that contribute little to meeting dietary recommendations). Bottom line — it depends on how you measure their price! See http://www.ers.usda.gov/publications/eib96/.
Perhaps flowers, shrubs, and trees are also perceived as expensive relative to other things that folks might spend their disposable income dollars on, particularly if the end consumer perceives them as mere interior and exterior landscape enhancements. But if all of the economic, environmental, and health/well-being benefits are considered, then plants are the best bargain going. Again, it depends on how you measure their price! See http://www.americainbloom.org/resources/Discover-Plants-Brochure-and-Presentation.aspx.