This month, a report in the Journal of the American Dietetic Association* compared foods on cost per calorie as well as cost per nutritional density. (This was done on prices primarily in France, with correlations to overall U.S. prices. At this time the authors are working on conducting a similar study using food prices in various U.S. cities.)
The upshot is that when you compare cost on a nutritional basis that is the cost of getting all the vitamins, minerals, and other essential nutrients needed in our food the best buys are vegetables and fruits, with lean meat and low fat dairy not far behind**. This is very good news for followers of reduced-carbohydrate diets, because it implies that getting adequate nutrition need not be overly expensive.
The trouble is that on a per calorie basis, the starchy and sugary foods are much less expensive, while being among the poorest sources of nutrition per calorie. This, of course, is one of the main reasons that its hard for people with lower incomes to get adequate nutrition when money is tight, people are going to sacrifice nutrition for calories. (Also, in some lower income areas, fresh foods are more difficult to obtain and transport.)
What is the answer for low carbers? Focus mainly on the high nutrition foods, most of which are low in carbohydrates. Then, fill in the rest of your calorie needs with good fats. Olive oil, for example, is being shown to have nutritional benefits as well as being a source of healthy monounsaturated fat.
*Darmon N, Darmon M, Maillot M, Drewnowski A., A nutrient density standard for vegetables and fruits: nutrients per calorie and nutrients per unit cost. Journal of the American Dietetic Association, Dec. 2005 105(12):1881-7. Abstract
**From communication with author