MWAA has to get special permission from the airline signatories to the lease agreement to put DCA money to IAD - which they have gotten in the recent past - but it would be only a relatively small amount. At the same time, anything that gets the enplanement costs down, which has gone down significantly over the last 8 years or so, makes a difference.
MWAA recently sold the western lands that really have no use for air operations and that money is being used to finance several improvements and bring in interest/investment income. In addition, MWAA will be developing at least one new hotel on the property. There are other ideas/plans being discussed (a professional sports stadium was for a short, very short, while).
There is a ginormous, untapped cash cow at IAD, that would make the place super cost-competitive, and that is commercializing the completely unused historic tower. I was recently up in it, and it has so much potential (for restaurant, bar, observation lounge, etc.). Unfortunately, the existing elevator is not ADA compliant and making it compliant would require substantial alteration in violation of historic preservation regulations. Various ideas have been batted about, but pretty much, without an act of Congress (who also actually owns the place) nothing can/will ever happen. It's a huge shame.