The typical asking rate per listing is in the $30's per square foot per year. The Napa submarket is going through a decrease in rates. In the last 3 months, the median rate fell $1 per sqft per year. That's a 4% decrease in rent.
In the last 90 days, 34 listings have been posted online, which means office spaces most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 29% of the available leases in the Napa are subleases, so you might be able to snag a great spot on a short term. 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 2,400 sqft. Most buildings in this area are around 1 floors. Most buildings in this area were built in 1987 and are Class A.