The typical asking rate per listing is in the $20's per square foot per year. The Manassas region is going through a a boost in rates. In the last 3 months, the median rate went up $3 per sqft per year. That's a 14% increase in rent.
Compared to the rest of the Washington D.C. market, the Manassas submarket has less expensive rates, so you should have no trouble finding a great deal in this undervalued submarket.
In the last 90 days, 29 listings have been listed, which means listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. About 52% of the available leases in the Manassas are subleases, so you might be able to find a shorter term or below market rent. 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 2,500 sqft. Most properties in this region are around 2 floors. Most buildings in this area were built in 1989 and are Class B.