Another thought is renting a car. Avis, and maybe other rental companies, allow you to pick-up a car at DCA-IAD-BWI and drop it off at any of the other airports at no additional charge. This could be cheaper than a taxi but will likely be more than public transportation. It may be $50 from downtown to BWI, but I think that is is closer to $75 from DCA-BWI. I don't know if priceline rentals may be returned to different airports than the one rented from, but if so it could be as little as $20 from DCA-BWI. I would gladly pay $20 to avoid 4 hours of public transport. Keep in mind that at rush hour, morning or evening, this trip could take 2 hours plus.