What transport alternatives are there between Dulles airport and Baltimore airport?

Ruling out car rental?

Ronald Reagan Washington Airport (DCA) (Washington DC has 2 airports)....

The cheapest is probably SuperShuttle:

SUPERSHUTTLE
-------------
DOOR-TO-DOOR SHARED RIDE VAN. SUPERSHUTTLE'S DOOR-TO-DOOR SERVICE IS
AVAILABLE TO THE FLYER COACH STOPS, AS WELL AS UNION STATION.

SUPERSHUTTLE STOPS ARE CLEARLY IDENTIFIED ON THE GROUND TRANSPORTATION
LEVEL ROADWAY OUTSIDE THE MAIN TERMINAL AT WASHINGTON DULLES.
SHUTTLES OPERATE ON A SHARED RIDE-ON DEMAND BASIS.
FOR INFORMATION, CALL 1-800-BLUEVAN OR GO TO WWW.SUPERSHUTTLE.COM

Super Shuttle is one option - a shared ride shuttle from one to the other. There are a couple, and they probably run around $45 or $50


Reston Coach, and other Limo services have transport. Reston Coach runs $145

Washington Flyer Taxi will handle it for some amount between those two.

It is possible to do this via public transport two ways, depending on what day of the week it is.

A. You can take the MARC train (about $8) to Union Station, then the metro to either East Falls Church (where you pick up a Washington Flyer Bus) or to L'Enfant Plaza or Rosslyn (where you pick up a 5A Bus) MARC Train only runs Monday to Friday. You'll pay about $20 total for this.

B: You can take a bus from BWI (Bus is the B30 toward Greenbelt.) Get on the green line metro at greenbelt and take it to L'Enfant Plaza, where you will pick up the 5A bus to Dulles. This costs $8.20 total but takes three hours.

You can take the local 5A bus from Dulles to the Rosslyn metro station. From there, take the train to the L'Enfant Plaza stop and transfer to the green line. Take that train to the Greenbelt stop (end of the line) and from there you can take the express bus to BWI airport.