By public transportation: take Airtrain from your terminal to Jamaica station. Then take E subway to Lexington Ave. and transfer to the downtown/Brooklyn bound 4,5 or 6 subway to 42nd St./Grand Central. Walk one block west to Madison Ave. and then turn left. Cost is $7.25 and time frame is a little over one hour.
Taxi from JFK is a flat rate of $50.50 ($45 plus $5 toll and 50 cent surcharge) and time frame is 35-60 minutes depending on time of day and traffic.
By public transportation: