The typical asking rate per listing is in the $20's per square foot per year. The Manassas region is going through a decline in rates. In the last 3 months, the median rate fell $2 per sqft per year. That's a 10% decrease 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 region.
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 tour spaces as quick as possible. About 7% of the available leases in the Manassas are subleases, so you will probably have to consider a longer term lease. 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 2,190 sqft. Most properties in this region are around 2 floors. Most buildings in this area were built in 1989 and are Class B.