Most listings are in the $50's per square foot per year. The O'Fallon submarket is going through a a boost in rates. In the last 3 months, the median rate went up $22 per sqft per year. That's a 61% increase in rent.
In the last 90 days, 9 listings have been posted online, which means listings most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 33% of the listings in the O'Fallon are subleases, so you might be able to find a shorter term or below market rent. 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 120 sqft. Most properties in this region are approximately 1 floors. Most buildings in this region were built in 1999 and are Class B.