The typical asking rate per listing is in the $30's per square foot per year. The Ontario region is going through a a rise in rates. In the last 3 months, the median rate increased $2 per sqft per year. That's a 6% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Ontario submarket has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 64 listings have been listed, so you'll want to check back regularly for new listings. About 3% of the office spaces in the Ontario are subleases, so you will probably have to consider a longer term lease. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 2,120 sqft. Most properties in this region are around 2 stories. Most buildings in this market were built in 2001 and are Class A.