Most listings are in the $40's per square foot per year. The Napa submarket is experiencing a a boost 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 leases most likley are leasing quickly. 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 region are around 1 floors. Most buildings in this region were built in 1983 and are Class A.