Most listings are in the $20's per square foot per year. The River Forest region is seeing a a boost in rates. In the last 3 months, the median rate went up $24 per sqft per year.
Compared to the rest of the Chicagoland market, the River Forest region has less expensive rates, so you should have no trouble finding a great deal in this undervalued region.
In the last 90 days, 1 listings have been listed, which means new listings 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,500 sqft.