One thing he mentions is that especially in climates with short growing seasons, local farmers are scrambling to make a year's income in only 3-4 short months of harvest. Therefore, unlike the supermarket (read: California) farmers who have most of the year to get a return on their investment, they have to charge a premium price during those precious summer months.
In Animal, Vegetable, Miracle, Barbara Kingsolver offers a way to help your local farmers out and therefore encourage them to return to the market the next season. Instead of just buying what you need for the week or for the summer, buy a bunch of extra tomatoes, peaches, berries, garlic, melons, herbs and such, and can or freeze them for use during the winter. That way when you want to make a tomato sauce or smoothy in December, you can reach for your stored produce. I actually bought a bunch of extra pints of berries this weekend and froze the extras--they'll be much better yogurt mix-ins than the bland supermarket option in a couple of months. And now that farmer my local farmer has that $10, instead of Key Foods. And right now those berries, tomatoes, squash and melons are much cheaper at the Farmer's Market than they will be out of season in the grocery store in a few months--so in this case you can actually save money in the long run and end up with a better meal.