The typical asking rate per listing is in the $30's per square foot per year. The Napa region is seeing a an increase in rates. In the last 3 months, the median rate went up $3 per sqft per year. That's a 8% increase in rent.
Compared to the rest of the Bay Area market, the Napa submarket has less expensive rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 22 listings have been made available, which means spaces are probably leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,530 sqft. Most properties in this region are close to 1 stories. Most buildings in this market were built in 1980 and are Class B.