Sam over at Becks and Posh did a little comparison shopping, and discovered to her surprise that by shopping at the Farmer’s Market last weekend, she saved 29% over what it would have cost her to buy the same items at the supermarket. Considering that she was shopping at Ferry Plaza Market, what’s so exciting about this is that Sam’s also been keeping track of her food expenditures all year — and what she’s finding is that for ordinary produce shopping she’s ahead by going to the market. I’ve shopped Farmer’s Markets for 20 years (scary, that thought — I’m…