BWI-DCA is $39/1st person + $10/each additional person (they shuttle people in the mini van). It is http://www.supershuttle.com/ if you are interested. They are only $20 (+$10 each addl person) for DCA to IAD if you ever have to do that.

Also, you can take Amtrak from BWI rail to Washington Union Station, then you can take the subway from union station to DCA. I think BWI Rail is like a mile from the BWI Air terminals, I'm not sure what the ground transportation is like to get to it. You might find a tram, have to walk, or at worst pay a cab if you are watching the $. But, it can't cost much to get there. The train ride is $9, you need reservations. Then, I haven't taken the metro in a while, but I can't imagine the fare would be any more then $3 from Union station to DCA. Still under $20 if you had to take a cab from BWI to BWI rail. Maybe someone else can explain how to get between the two.

-Jason