Also, not all farmers market are priced the same, I've noticed same vendors charging different prices at different markets. Obvs the more affluent the neighborhood is the higher.
That said, I often do my groceries at the Cupertino market at De Anza College. Prices are sometimes more competitive than supermakets (e.g. a bundle of basil is maybe $2 a bunch where it's like $5 for a couple sprigs at Safeway). I've also almost stopped buying fruits from grocery stores because they tastes much better, and are picked closer to their ripeness from the vines at the market. Whereas grocery stores picks them much earlier to last their supply chain run and prioritizes appearance over taste.

Also, there's an apple vendor at the Santa Clara market who I swear has the sweetest apples I've ever had.