I believe that the same scholarships are available to both the RD and ED groups; i doubt that there are particular merit scholarships reserved for ED applicants (although with the recent scholarship/financial aid initiative this could be changing). So you may have to wait until spring to find out about merit scholarships.
I'm pretty sure that "inability to pay" is considered an acceptable reason to back out of an ED contract, so if you talk to WUSTL about it beforehand, you could probably apply RD to another school or two, in case financial aid and merit scholarships aren't going to be enough... best of luck
