Megabus or Boltbus from DC to NY-Penn Station is about $25, Amtrak is $45 and up but genereally faster than bus. From Penn Station; taxi is $45+tolls+tip, NYAS express bus is $15, LIRR to Jamaica and AirTrain to terminals is $$13/$10.75 off-peak, or E subway to Sutphin Blvd/JFK and then AirTrain is $7.25.
Megabus or Boltbus from DC