I'd second the Mondavi for being a consistently good Napa cab for 30 bucks.
Usually in that price range I don't look to Napa. There's some excellent ones you might enjoy from Chile though, which is my fallback for great value Cabs. Something like Concha Y Toro's Marques de Casa Concha (a favorite of mine for 18 or 20 dollars) and the Montes Alpha.
And if you liked that Malbec try the Tilia Malbec and Cabernet. Only about 10 bucks and both excellent from Argentina.