Most listings are in the $30's per square foot per year. The Napa submarket is going through a decrease in rates. In the last 3 months, the median rate fell $18 per sqft per year. That's a 32% decline 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 undervalued region.
In the last 90 days, 22 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,400 sqft. Most properties in this area are around 1 stories. Most buildings in this area were built in 1980 and are Class B.