Most listings are in the $30's per square foot per year. The San Anselmo submarket is seeing a an increase in rates. In the last 3 months, the median rate rose $10 per sqft per year. That's a 32% increase in rent.
Compared to the rest of the Bay Area market, the San Anselmo submarket has lower rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 6 listings have come on the market, which means listings are probably leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,400 sqft. Most buildings in this area were built in 1962 and are Class B.