FYI-be somewhat careful of your hotel choice. I was born and raised in the Washington area and I would personally avoid hotels in NE Washington, DC as well as anything in Prince George's Co, MD (where Oxon Hill is) for safety purposes. Case in point, there are several hotels on New York Avenue in NE DC and I see tourists all the time walking to and from their hotels looking PRETTY uncomfortable... You won't see THAT info on the hotel brochures.
That being said, we've always had luck with priceline, but we seem to get the best deals when we plan a last-minute trip.
Also, there is no subway from Dulles airport to DC (not planned until 2016 or so-if at all). The Washington Flyer bus is the "approved" bus that you can take, but I'd bet it is significantly more than $1.50 these days.
Good luck planning your trip. April is a great time to be here (if this cold weather ever breaks)!