What about BWI?

THere's a cheaper option at £83 now out of BWI compared to a minimum of £100 ($155?) out of DCA and IAD.
Seems to be about 25-30 mins by Amtrak to BWI for $11 (standard coach seat) or maybe $6 by a bus (vs $66 by cab)

Think Union Station is only a few stops from L'Enfant Plaza station which is the nearest to me. Not exactly sure which terminal American Airlines flies out of as some forms of transport have a couple of stops.