Most listings are in the $20's per square foot per year. The Fountain Valley region is experiencing a a rise in rates. In the last 3 months, the median rate rose $2 per sqft per year. That's a 7% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Fountain Valley submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 29 listings have been made available, which means leases most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. 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 1,000 sqft. Most properties in this area are around 3 floors. Most buildings in this market were built in 1982 and are Class B.