To revive the posting on affordable bus service to/from DC area to BWI, this is the link to the schedule for the Express Bus that runs from the Greenbelt Metro stop to BWI for $2... about every half hour during metro operation times.

To people that aren't from around DC, this means take the Bus to the Metro, then Metro to DC area.... downtown for roughly $5 and in about 45 mins from when you board the bus.

http://www.wmata.com/timetables/md/B30.pdf

I personally use the MARC Train service which is $5 or $2 (3? for Seniors). This is by far the most reliable way to get you to BWI as the trains are generally on time and don't get stuck in the really bad traffic that the DC/Baltimore corridor suffers from during rush hour.

MARC Schedule:
http://www.mtamaryland.com/schedules...mtrak_Penn.pdf