Tim Meineke,
You can get to LGA more cheaply than that:
Take the E or F train to Jackson Heights - Roosevelt Avenue. Go upstairs and get a free transfer (with MetroCard) to the Q33 bus to the main terminals at LGA or the Q47 bus to the Delta Shuttle terminal. The subway from Manhattan is 15-30 minutes. The bus is another 20-30 minutes. All for $1.50.
Bruce