On average, listings lease in the $20's per square foot per year. The O'Fallon submarket is experiencing a an increase in rates. In the last 3 months, the median rate increased $2 per sqft per year. That's a 10% increase in rent.
In the last 90 days, 31 listings have been posted online, which means listings most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. About 3% of the available leases in the O'Fallon are subleases, so you will probably need to sign a longer term lease with the landlord directly. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,360 sqft. Most properties in this area are close to 2 floors. Most buildings in this area were built in 2001 and are Class A.