Shake, the last time I went to NYC with my mother, my friend & her mother we got from LaGuardia to our hotel in Manhattan with this 'shuttle' service (I'm not sure exactly how legal it was or anything). The man's name was Peter (black with long dreadlocks) & he was very friendly; he charged $15 per person, got us right to the door of our hotel with no problem, provided commentary & answered any questions we had. The phone # I have is 1-281-541-9506, but the number may have changed since then.
I think the cabs tell you how much it is up front, my travel guide says it should be $35-40. New York Airport Express Bus costs $13 one way, $21 round trip.