The typical asking rate per listing is in the $30's per square foot per year. The Napa submarket is experiencing a an increase in rates. In the last 3 months, the median rate increased $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 region.
In the last 90 days, 23 listings have come on the market, which means new availabilities most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,440 sqft. Most properties in this region are close to 1 floors. Most buildings in this area were built in 1980 and are Class B.