Unfortunately, easiest rarely equals cheapest. The absolute cheapest way by public transportation is to get to Terminal 4 (you can ride the AirTrain between terminals free of charges), and then take to Q10 or Q10 Limited bus to the end of the line (Kew Gardens) for $2.25. From Kew Gardens, take a Manhattan-bound E train to Penn Station. If you purchase a Metrocard at JFK before boarding the bus, you will get a free transfer from the bus to the subway; otherwise you will have to pay a separate subway fare ($2.25 or $2.50, depending on whether you purchase a Metrocard or a single-ride ticket at the subway station).
Unfortunately, easiest