Cheapest is by Boltbus or Megabus from Penn Station, around $25 one way. Much cheaper than train but not as fast. Cheapest route to Penn Station is AirTrain from terminal to Jamaica Station and then E subway to Penn Station, total cost is $7.25. Buses are outside Penn Station at 7th Ave/33rd St. Amtrak will cost you minimum $45 depending on time of day you depart.
Cheapest is by Boltbus or