The typical asking rate per listing is in the $20's per square foot per year. The Riverside submarket is going through a a boost in rates. In the last 3 months, the median rate rose $2 per sqft per year. That's a 6% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Riverside submarket has less expensive rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 79 listings have been posted online, so you'll want to check back regularly for new listings. About 1% of the available space in the Riverside 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,630 sqft. Most buildings in this area are around 2 stories. Most buildings in this region were built in 1989 and are Class C.