Most listings are in the $10's per square foot per year. The Greenbelt submarket is going through a decrease in rates. In the last 3 months, the median rate fell $15 per sqft per year. That's a 48% decrease in rent.
Compared to the rest of the Washington D.C. market, the Greenbelt submarket has lower rates, so you should have no trouble finding a great deal in this depreciated submarket.
In the last 90 days, 10 listings have been posted online, which means new availabilities 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 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 2,000 sqft. Most buildings in this area are close to 8 floors. Most buildings in this market were built in 1986 and are Class A.