AMTRAK takes you to Penn Station in midtown. Cheapest shuttle fares I have seen are $116.50 on a weekend, but I think the latest departure time at this price is 2PM, and of course you have to get from LaGuardia to M and back again. This could take 30 min-1 hour depending upon mode of travel and traffic. SO - the "cheapest" way would depend upon where you are meeting your party and how long you want to spend with them.