On average, listings lease in the $10's per square foot per year. The Orange County region is going through a an increase in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 11% rise in rent.
In the last 90 days, 25 listings have been made available, which means office spaces 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,030 sqft. Most buildings in this area were built in 1998 and are Class B.