On average, listings lease in the $30's per square foot per year. The Diamond Bar submarket is going through a a rise in rates. In the last 3 months, the median rate increased $32 per sqft per year.
Compared to the rest of the Metropolitan Los Angeles market, the Diamond Bar region has lower rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 23 listings have been made available, which means leases most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 48% of the available space in the Diamond Bar are subleases, so you might be able to snag a great spot on a short term. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
Even though the median size is 2,630 sqft, there are still 0 new listings below 2,000 sqft. Most properties in this market are close to 2 stories. Most buildings in this area were built in 1987 and are Class B.