The typical asking rate per listing is in the $20's per square foot per year. The Covina region is experiencing a an increase in rates. In the last 3 months, the median rate went up $6 per sqft per year. That's a 29% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Covina submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 10 listings have been made available, which means listings most likley are going fast. You'll want to be prepared to make an offer immediately. 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 2,270 sqft. Most properties in this market are around 2 stories. Most buildings in this area were built in 1981 and are Class B.