Only a crazy person would EVER recommend IAD over DCA when going to Baltimore. As a DC resident, always go with DCA! And a regular Amtrak train (Acela Regional Service) (but NOT the Acela Express) from DC to Baltimore runs $18, NOT $50. The MARC train is definitely cheaper and definitely slower as well. It is also much grungier than Amtrak. I live here and I know for sure. For the extra few bucks, I take Amtrak and also get AGR points as well.

The MARC trains don't run on the weekends. Make sure to check the schedule so that it lines up with your travel plans.