On average, listings lease in the $20's per square foot per year. The Ballwin submarket is going through a a boost in rates. In the last 3 months, the median rate rose $5 per sqft per year. That's a 27% increase in rent.
In the last 90 days, 6 listings have been made available, which means new availabilities most likley are going fast. You'll want to be prepared to make an offer immediately. 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 2,030 sqft. Most buildings in this market are around 3 stories. Most buildings in this market were built in 1972 and are Class B.