On the flip side, whenever I go to local farms, I find the prices to be high. I am not sure why this happens. For example, a pound of tomatoes just the other day at a local farm was $5.99 and my local Whole Foods was selling organic tomatoes for $2.99. I also can never seem to find organic produce from local farms.
At a guess, your local farm is engaging in price discrimination. If you care enough about "buy local" to drive out to the farm to buy the vegetables, chances are they can charge you more.
I do wonder how often small farms hide price discrimination behind "economies of scale" though.