Most listings are in the $20's per square foot per year. The Fullerton submarket is going through a an increase in rates. In the last 3 months, the median rate rose $3 per sqft per year. That's a 12% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Fullerton submarket has lower rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 30 listings have been made available, which means office spaces might be 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 800 sqft. Most properties in this area are around 2 stories. Most buildings in this market were built in 1975 and are Class B.