The typical asking rate per listing is in the $20's per square foot per year. The Riverside submarket is going through a an increase in rates. In the last 3 months, the median rate rose $2 per sqft per year. That's a 6% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Riverside submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 92 listings have been posted online, so you'll want to check back regularly for new listings. About 1% of the available leases in the Riverside are subleases, so you will probably have to consider a longer term lease. 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 2,140 sqft. Most properties in this area are approximately 2 floors. Most buildings in this region were built in 1989 and are Class C.