The typical asking rate per listing is in the $30's per square foot per year. The Napa region is experiencing a decrease in rates. In the last 3 months, the median rate decreased $23 per sqft per year. That's a 39% decrease in rent.
Compared to the rest of the Bay Area market, the Napa submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 21 listings have been posted online, which means new listings are probably 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,420 sqft. Most properties in this area are close to 1 floors. Most buildings in this area were built in 1980 and are Class B.