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The absolute cheapest way is to take the AirTrain to Lefferts Blvd., then the Q10 bus to Queens Blvd. ($2.90), then the E train to Penn Station (free transfer), then the NJT to Newark Airport Station (not sure how much $$), and then the EWR AirTrain.
Word of caution, that it takes forever.
Budget at least 1.30-2 hours.