The typical asking rate per listing is in the $20's per square foot per year. The Brentwood region is going through a an increase in rates. In the last 3 months, the median rate rose $25 per sqft per year.
In the last 90 days, 10 listings have been made available, which means office spaces most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 50% of the office spaces in the Brentwood are subleases, so you might be able to sign a shorter term by taking over a previous lease. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
Even though the median size is 10,000 sqft, there are still 0 listings below 2,000 sqft. Most properties in this area are close to 3 stories. Most buildings in this area were built in 1965 and are Class B.