The typical asking rate per listing is in the $50's per square foot per year. The Mill Valley region is going through a a rise in rates. In the last 3 months, the median rate increased $4 per sqft per year. That's a 8% rise in rent.
Compared to the rest of the Bay Area market, the Mill Valley region has more expensive rates, and you'll need to make a great offer to be in this hot region.
In the last 90 days, 11 listings have been listed, which means listings most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 9% of the office spaces in the Mill Valley are subleases, so you will probably have to consider a longer term lease. 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 250 sqft. Most buildings in this market are around 2 stories. Most buildings in this area were built in 1964 and are Class A.