How expensive what would be? A taxi from JFK to 31st/9th? $45+toll and tip is the flat rate for a taxi from JFK to Manhattan, total fare for up to 4 people. I would never take a cab for this trip, though...your destination is very close to Penn Station, so you could save substantial money by taking the $5 AirTrain to Jamaica station and an LIRR train for about $9 to Penn.
How expensive what would be?