On average, listings lease in the $20's per square foot per year. The Brentwood submarket is seeing a a boost in rates. In the last 3 months, the median rate increased $22 per sqft per year.
In the last 90 days, 2 listings have been posted online, which means leases most likley are going fast. You'll want to be prepared to make an offer immediately. 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 1,670 sqft.