On average, listings lease in the $50's per square foot per year. The O'Fallon submarket is going through a an increase in rates. In the last 3 months, the median rate increased $22 per sqft per year. That's a 61% rise in rent.
In the last 90 days, 9 listings have been listed, which means new listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 33% of the available leases 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 on the market, 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 buildings in this area are approximately 1 floors. Most buildings in this market were built in 1999 and are Class B.