the cheapest is take Metro-North to NYC (Grand Central) and once you get out, across the street (on Park Ave between 41st and 42nd st) there is express bus service to LGA (Olympia). Cost is $12 for the bus and about $15 (??) for Metro-North. Good if you have time, low on money and not a lot of luggage. Overall, more time consuming that driving direct without traffic (a big IF). If driving, tell us what time is your flight? Traffic can be very very bad at times, even before you get to 287 turnoff. I drive Milford-NYC regularly, and at times it takes me 2.5hrs to go Milford-Greenwich (regular traffic, not accidents).