Most listings are in the $10's per square foot per year. The Montclair region is experiencing a a boost in rates. In the last 3 months, the median rate increased $20 per sqft per year.
Compared to the rest of the Metropolitan Los Angeles market, the Montclair submarket has lower rates, so you should have no trouble finding a great deal in this discounted submarket.
In the last 90 days, 0 listings have been posted online, which means leases are probably leasing quickly. You'll want to be prepared to make an offer immediately. About Inf% of the available space in the Montclair are subleases, so you might be able to sign a shorter term by taking over a previous 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 1,030 sqft. Most buildings in this region were built in 1986 and are Class B.