Most listings are in the $20's per square foot per year. The Walnut submarket is going through a an increase in rates. In the last 3 months, the median rate went up $23 per sqft per year. That's a Inf% rise in rent.
Compared to the rest of the Metropolitan Los Angeles market, the Walnut submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 1 listings have been posted online, which means spaces might be 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 960 sqft. Most buildings in this area were built in 1987.