The typical asking rate per listing is in the $10's per square foot per year. The Riverdale region is going through a an increase in rates. In the last 3 months, the median rate rose $15 per sqft per year.
Compared to the rest of the Metro Atlanta market, the Riverdale region has smaller rates, so you should have no trouble finding a great deal in this discounted region.
In the last 90 days, 3 listings have been posted online, which means spaces are probably 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 1,000 sqft. Most buildings in this area are around 1 stories. Most buildings in this region were built in 2003 and are Class B.