I grew up working (grudgingly) in my parent's garden. Every year, as I hilled potatoes, I wondered why we grew the same vegetables that were on sale dirt cheap in the grocery store. I still wonder that as I see people plant zucchini, tomatoes and green beans, veggies that are practically given away during the height of the season.
Instead, perhaps gardeners should concentrate on those items that they pay a hefty price for. If I were planning a garden strictly on saving the most money, I'd include--
Yellow and red peppers: no harder to grow than green ones, and sliced and bagged, they freeze well.
Basil, cilantro, and parsley: A friend makes his pesto in huge batches and freezes it in ice cube trays, a perfect one-meal size. The cost of buying fresh herbs is many times its cost to grow.
Brussels Sprouts: An easy crop to grow and infinitely better fresh than frozen, the price never seems reasonable at the grocery store.
Jalapeno and serrano peppers: Also sold at a premium, these will last a long time in the crisper, especially if stored in bags specially made to prolong the life of greenstuff. Note: keep them well away from your other peppers, so they don't cross-pollinate and provide a hot surprise.
Sugar snap peas: another vegetable easy to grow but expensive to buy, it is delicious raw or cooked.
What veggies could you save the most money on by growing yourself?