On average, listings lease in the $40's per square foot per year. The Napa region is going through a an increase in rates. In the last 3 months, the median rate increased $4 per sqft per year. That's a 12% rise in rent.
In the last 90 days, 34 listings have been made available, which means 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,740 sqft. Most properties in this area are around 1 stories. Most buildings in this market were built in 1983 and are Class A.