Baltimore BWI and Dulles are both very popular east coast hubs, and in my experience you can often find good fares from the West Coast to BWI when other Airports are as much as twice the price. Although there is an inconvenience factor you can get back and forth between these two airports either with a dedicated taxi or shuttle at about $75-$95 or, if you are willing to be a little adventurous, try this which to me seems the best option all things considered and costs only about $15:
OR you could take the Metrorail to Union Station board a northbound MARC Penn Line train to BWI.
This is from the Washington Area Airports page:
The Airports Authority does not provide a dedicated shuttle between the airports.
Below are options between Dulles and Baltimore Washington International Airport.
Direct travel: Washington Flyer taxi (estimated fare $97.00 and 60-70 minutes travel time) or SuperShuttle shared ride van service (estimated fare $75.00 and 70-90 minutes travel time) are available between Dulles and BWI. The other options may require connecting to other modes of transportation to get to your final destination.
- Other travel modes: You can also use Washington Flyer Taxi (estimated fare $30.00 and 15-20 minutes travel time) or SuperShuttle shared ride van service (estimated fare $20.00 and 20-30 minutes travel time) or Washington Flyer Coach (estimated fare $10.00 and 15-20 minutes travel time) for ground transportation to the West Falls Church Metro station. From there, you could take the Metrorail to the Greenbelt Metro station ($3-$5) then board a BWI Express Metro bus. You can also take the Metrorail to Union Station board a northbound MARC Penn Line train to BWI.
Baltimore City via public transportation
You use the Washington Flyer Coach (estimated fare $10.00 and 15-20 minutes travel time) for transportation from Dulles to the West Falls Church Metrorail station. From there, you could take the Metrorail to Union Station, where you can board a northbound MARC train to Baltimore.