Most listings are in the $20's per square foot per year. The Riverside region is going through a a rise 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 region has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 74 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 posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,600 sqft. Most buildings in this market are close to 2 floors. Most buildings in this market were built in 1989 and are Class C.