Most listings are in the $10's per square foot per year. The Brentwood submarket is going through a a rise in rates. In the last 3 months, the median rate rose $17 per sqft per year.
In the last 90 days, 10 listings have been made available, which means listings most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. About 10% of the available leases in the Brentwood are subleases, so you will probably have to focus your search on direct office leases. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
Even though the median size is 10,000 sqft, there are still 0 new listings below 2,000 sqft. Most buildings in this area are approximately 3 floors. Most buildings in this market were built in 1965 and are Class B.