Most listings are in the $30's per square foot per year. The Napa region is experiencing a a rise in rates. In the last 3 months, the median rate rose $6 per sqft per year. That's a 22% increase in rent.
In the last 90 days, 21 listings have been posted online, which means office spaces most likley are going fast. You'll want to be prepared to make an offer immediately. 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 1,320 sqft. Most properties in this region are around 1 floors. Most buildings in this market were built in 1983 and are Class A.