Yes the further south or west of DC you go it gets cheaper. The closer to DC you are, the closer you are to Metro stops, and the rent goes up up up.
Manassas is not bad, I lived for 2 years in an apartment complex called Battery Heights. They are older apartments, but its a lot of space for the $$, At the time when I moved there, it was the nicest apartment I could find at the price I wanted to pay. One of my co-workers just moved there actually. There is a shopping center/grocery store within walking distance and even a Wal Mart if you feel like crossing 6 lanes of traffic. Best to consider that (and a lot of other shops) a short drive away.
I currently work in Manassas, but now I live about 45 miles south. The mortgage on my house is about $100 more than my rent was in Manassas. So yea if you are willing to commute a long distance, things get cheaper.
Maybe consider Dale City/Woodbridge area? Its near the 95 corridor so that would be good for getting into the DC area. You know, except for traffic.