Most listings are in the $20's per square foot per year. The Ontario region is seeing a decline in rates. In the last 3 months, the median rate fell $4 per sqft per year. That's a 13% decrease in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Ontario region has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 48 listings have been posted online, so you'll want to check back regularly for new listings. About 4% of the available leases in the Ontario are subleases, so you will probably need to sign a longer term lease with the landlord directly. 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 1,920 sqft. Most properties in this region are close to 2 stories. Most buildings in this market were built in 2002 and are Class A.