On average, listings lease in the $10's per square foot per year. The Affton submarket is going through a a boost in rates. In the last 3 months, the median rate went up $17 per sqft per year.
In the last 90 days, 2 listings have been posted online, which means spaces most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. 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 1,300 sqft. Most properties in this market are around 2 floors. Most buildings in this area were built in 2001 and are Class B.