Most listings are in the $30's per square foot per year. The Brea submarket is going through a a rise in rates. In the last 3 months, the median rate went up $2 per sqft per year. That's a 5% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Brea region has less expensive rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 14 listings have been posted online, which means new listings most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 7% of the listings in the Brea 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.
Even though the median size is 2,880 sqft, there are still 0 leases below 2,000 sqft. Most properties in this area are close to 2 stories. Most buildings in this area were built in 1985 and are Class B.