Most listings are in the $30's per square foot per year. The Ontario region is seeing a a rise in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 3% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Ontario region has less expensive rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 48 listings have been made available, so you'll want to check back regularly for new listings. About 4% of the available space 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,840 sqft. Most properties in this market are around 2 stories. Most buildings in this area were built in 2001 and are Class A.