If you are looking at VERY low budget, I suggest the Sleep Inn in Rockville, MD just off of Shady Grove Rd.
It is located in a business park, so there isn't anything 'scary' about the area. They offer shuttle service, OR you can just drive to the Shady Grove Metro station.
I stayed there for the first time in 2004, and then again last year - and the property is nice, safe, and well maintained. In 2004 I paid less than $50 a night - it's up to around $79 or so on average I think.
As the PP said - King St. is nice! Lots to do, and it is very pedestrian friendly! So much to see and do in Alexandria, you might not want to go in to DC! HAHA
Adjacent to the King Street Metro there is in fact an Embassy Suites, Hampton Inn, and a Hilton. There is a Wyndham timeshare property there as well - it shares a the corner lot with the Hilton.
There is also a Kimpton hotel in the area - sometimes you can get decent prices at Kimpton properties through websites like snique away and groupon getaways.
Silver Spring is also a very convenient location - there is a Double Tree, Hampton Inn, and Homewood Suites there. The Double Tree has its own parking garage - the other two adjacent properties share parking areas and have access to the city owned parking garage next door.
I hope that helps!
I hope that helps!