Most listings are in the $50's per square foot per year. The Napa submarket is going through a a rise in rates. In the last 3 months, the median rate went up $12 per sqft per year. That's a 30% rise in rent.
Compared to the rest of the Bay Area market, the Napa submarket has greater rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 23 listings have been made available, which means office spaces are probably going fast. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 920 sqft. Most buildings in this region are approximately 1 stories. Most buildings in this market were built in 1980 and are Class B.