Most listings are in the $30's per square foot per year. The Napa submarket is going through a a boost in rates. In the last 3 months, the median rate went up $4 per sqft per year. That's a 11% rise 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 undervalued region.
In the last 90 days, 23 listings have come on the market, which means office spaces might be 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 1,440 sqft. Most properties in this area are close to 1 floors. Most buildings in this region were built in 1980 and are Class B.