Most listings are in the $50's per square foot per year. The Mill Valley submarket is going through a a rise in rates. In the last 3 months, the median rate rose $55 per sqft per year. That's a Inf% increase in rent.
Compared to the rest of the Bay Area market, the Mill Valley region has greater rates, and you'll need to make a great offer to be in this hot submarket.
In the last 90 days, 9 listings have come on the market, 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 530 sqft. Most buildings in this area are around 2 stories. Most buildings in this market were built in 1964 and are Class A.