Reported in the Boston Globe, the American public radio program The Splendid Table is doing a one-year study on eating sustainably. It asked 14 participants across the country to keep track of what they spend on a local food diet -- 80 percent of their food items are from organic, regional sources within 500 miles of their homes. Seven out of 14 participants reported spending less on food. Five said they spent the same as usual. (Which, according to the Globe, might be less anyway given the rise in food prices.) The fate of the other two participants wasn't reported.
I reckon I spend way less on food by eating locally, simply because I'm buying almost no processed or exotic foods, which usually cost more. I often buy in bulk (lentils, beans), direct from the farmer (eggs, beef), when it's in season, abundant and less expensive (strawberries, asparagus) and harvest my own small garden (herbs, tomatoes). It's a little more work, but it saves a few bucks!