Most listings are in the $10's per square foot per year. The Irmo region is experiencing a an increase in rates. In the last 3 months, the median rate rose $1 per sqft per year. That's a 8% rise in rent.
In the last 90 days, 3 listings have been listed, which means office 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 available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,900 sqft. Most buildings in this market were built in 2007 and are Class B.