On average, listings lease 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 $33 per sqft per year.
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. About 26% of the listings in the Napa are subleases, so you might be able to find a shorter term or below market rent. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
Even though the median size is 2,990 sqft, there are still 0 office spaces below 2,000 sqft. Most buildings in this region are around 1 floors. Most buildings in this market were built in 1983 and are Class A.