On average, listings lease in the $30's per square foot per year. The Napa region is experiencing a a boost in rates. In the last 3 months, the median rate went up $4 per sqft per year. That's a 11% increase in rent.
In the last 90 days, 34 listings have come on the market, which means new listings 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 1,440 sqft. Most buildings in this region are approximately 1 stories. Most buildings in this market were built in 1983 and are Class A.