The typical asking rate per listing is in the $30's per square foot per year. The Ontario submarket is going through a a boost in rates. In the last 3 months, the median rate went up $2 per sqft per year. That's a 9% rise 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 submarket.
In the last 90 days, 64 listings have been made available, so you'll want to check back regularly for new listings. About 3% of the available space in the Ontario are subleases, so you will probably need to consider the implications of signing a direct lease with the building. 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 floors. Most buildings in this area were built in 2001 and are Class A.