American Airlines has roundtrip airfares to and from Washington DC area (BWI) and Miami (MIA) for $96 including taxes and fees. This is nonstop service.
Travel availability:
Departs BWI: Feb 7, 13, 27; Mar 13, 20
Departs MIA: Feb 8, 9, 14, 16, 23, ; Mar 2, 15, 16, 22, 23, 29
Book it on American Airlines
My sample itinerary below: