Are there any rural areas outside of Alexandria, VA with a lower cost of living? -

Yes, but they are pretty far out. If you are willing to live in a suburban area, prices drop somewhat when you get to Manassas and Woodbridge (about half an hour from DC) and then drop further towards Fredericksburg and Culpeper (about an hour).That close to DC, though? No chance.

There probably are, but you ll make up for it in gas money or train fare (and I think VRE only runs on weekdays and doesn t run on holidays) and time spent commuting.