Since you have an int'l flight at IAD you probably need to be there about 5pm. Baltimore to IAD is at least a 90 min trip on a Sat with no traffic. So I'd say your only real choice is a taxi or a shuttle service, both of which should be about $90 or so.

You could take Amtrak from Baltimore into DC for $11 one-way if you book in advance, then take a taxi from Union Station. A taxi from there will be about $60 so probably not worth doing that. A bus/subway combination would take you well over 2 hours so that's not a choice either.

Here's a site with a little more info:

http://www.mwaa.com/dulles/parking_t...travel_options