Most listings are in the $20's per square foot per year. The Glendora region is experiencing a a rise in rates. In the last 3 months, the median rate increased $6 per sqft per year. That's a 35% increase in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Glendora region has lower rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 4 listings have been made available, which means leases most likley are leasing quickly. You'll want to be prepared to make an offer immediately. 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 630 sqft. Most properties in this region are around 2 floors. Most buildings in this market were built in 1985 and are Class B.