I'm seeing the Hyatt Crystal City come up at US$76 on Sidestep for a Saturday night in April -- that's really good. Bookable through the Hyatt site.
But for a few more budget options, instead of Crystal City, try the north side of Alexandria proper, about five minutes south of DCA on the GW parkway.
For a Saturday night in April:
Alexandria Travelodge: $80
Holiday Inn Alexandria: $96
Sheraton Suites Old Town: $97
The Travelodge is a little dodgy but the others are great for a fast layover. Those prices are as good as you're going to get in this neighborhood w/o resorting to Priceline.
All quotes from Sidestep.com.