On average, listings lease in the $10's per square foot per year. The Fontana submarket is going through a an increase in rates. In the last 3 months, the median rate rose $2 per sqft per year. That's a 18% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Fontana region has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 10 listings have come on the market, which means office spaces are probably 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,250 sqft. Most buildings in this region are close to 1 floors. Most buildings in this area were built in 1994 and are Class A.