Most listings are in the $20's per square foot per year. The Darien region is going through a an increase in rates. In the last 3 months, the median rate increased $22 per sqft per year.
Compared to the rest of the Chicagoland market, the Darien submarket has smaller rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 5 listings have come on the market, which means new availabilities might be leasing quickly. You'll want to be prepared to make an offer immediately. About 120% of the listings in the Darien 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 500 sqft. Most properties in this market are around 1 floors. Most buildings in this area were built in 1990 and are Class B.