The typical asking rate per listing is in the $20's per square foot per year. The Brentwood region is going through a a boost in rates. In the last 3 months, the median rate went up $29 per sqft per year.
Compared to the rest of the East Bay market, the Brentwood region has lower rates, so you should have no trouble finding a great deal in this discounted submarket.
In the last 90 days, 5 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. About 180% of the listings 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 500 sqft. Most buildings in this market are approximately 1 stories. Most buildings in this region were built in 2003 and are Class A.