Most listings are in the $30's per square foot per year. The Napa region is going through a decline in rates. In the last 3 months, the median rate fell $16 per sqft per year. That's a 33% decline in rent.
Compared to the rest of the Bay Area market, the Napa region has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 23 listings have come on the market, which means office spaces are probably going fast. You'll want to be prepared to make an offer immediately. About 22% of the office spaces in the Napa are subleases, so you might be able to sign a shorter term by taking over a previous lease. 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 1,770 sqft. Most properties in this area are approximately 2 floors. Most buildings in this area were built in 1980 and are Class B.