The cheapest is by subway bus combo--which with any large bag is not too easy. Livery cabs (car service) may not be as cheap as a metered yellow cab. There are "Super Shuttles" but these are mainly geared for pickups in Manhattan. My rough guess is that from Prospect Park locale, $25 is the yellow cab rate--if you call a car service they can quote a price (since their rates are fixed in advance!) Have a nice flight.