Did you look into IAD at all? It seems to be between DCA and BWI in price most of the time, and it's got a few more options for getting into DC than BWI. In any case, like you said it's mostly a time versus money trade-off. It'll be 1-2 hours extra to go from BWI, depending on the bus schedule, if you take the B30. There's a pretty good outline of your options at http://dctransitguide.com/dc-airports/