The typical asking rate per listing is 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 went up $55 per sqft per year.
Compared to the rest of the Bay Area market, the Mill Valley region has greater rates, and you'll need to pay a premium to be in this hot region.
In the last 90 days, 11 listings have been made available, which means new listings are probably going fast. You'll want to be prepared to tour spaces as quick as possible. About 9% of the listings in the Mill Valley are subleases, so you will probably need to sign a longer term lease with the landlord directly. There aren't many listings below 2,000 square feet on the market, 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 properties in this market are close to 2 floors. Most buildings in this market were built in 1964 and are Class A.