Most listings are in the $10's per square foot per year. The Manassas Park County region is experiencing a a boost in rates. In the last 3 months, the median rate went up $20 per sqft per year.
In the last 90 days, 4 listings have been made available, which means new availabilities are probably leasing quickly. 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 region were built in 1946.