On average, listings lease in the $30's per square foot per year. The Napa submarket is experiencing a an increase in rates. In the last 3 months, the median rate increased $3 per sqft per year. That's a 8% rise in rent.
Compared to the rest of the Bay Area market, the Napa submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 10 listings have been listed, which means leases 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 1,530 sqft. Most properties in this market are around 1 floors. Most buildings in this region were built in 1978 and are Class B.