Napa ain't cheap.
But an aside. If you do the Silverado Trail (do!), contact Joseph Phelps Winery. If you do it far enough in advance you can book one of their wine blending seminars (be sure to specify that you're not just talking about a tasting session). You are given all the wines used to blend their premier wine "Insignia", and see if you can come close to what they do. It's also one of my favorite wineries in Napa.