The following are tips on how to not go into debt just to visit California's famed wine country. Sonoma County is definitely less expensive than Napa, but not cheap unless you're from NYC or Hawaii maybe. However, everything from lodging to wine tasting fees to wine purchases to eating out will be less expensive in Sonoma.
If you feel the need to spend time in Napa Valley, headquarter yourself next door in Sonoma County and spend a day of your trip in Napa. A few example drive times: Town of Sonoma to town of Napa, 25 minutes. Healdsburg to Calistoga in northern Napa Valley, 40 minutes. Santa Rosa to Calistoga, 30 minutes, to St. Helena in the heart of Napa, 45 minutes.