A relevant question is how much time as well as money do you have to spend?

It's possible (but not ultra convenient) to get from IAD to BWI using the Washington Metro system. Allow at least three hours for the transfer, and three and a half to four is probably more realistic.

1)-Take a #5A bus from Dulles to the L'Enfant Plaza Metrorail station.

2)-Take a Green Line Metrorail subway train from L'Enfant Plaza to Greenbelt Metrorail station

3)-Take a #B30 bus from Greenbelt Station to BWI

You can check timetables for the date and time you'd be making this trip by using Metro's "Rideguide" feature at:

http://rideguide.wmata.com/index.html

The IAD to BWI cost, according to Metro's website, would be $7.10.