The typical asking rate per listing is in the $40's per square foot per year. The Sonoma region is experiencing a an increase in rates. In the last 3 months, the median rate rose $41 per sqft per year. That's a Inf% increase in rent.
Compared to the rest of the Bay Area market, the Sonoma submarket has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 15 listings have come on the market, which means office spaces most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. 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 830 sqft. Most buildings in this region are close to 2 floors. Most buildings in this market were built in 1978 and are Class C.