Google Maps tells me Manhattan to JFK Airport is a 32 kilometre taxi ride and takes three hours, plus it takes two hours to clear security at the airport.
I'm a little astounded by those numbers. Five hours to get from the couch at home to the airport. Astounding. I'm through the airport and in Melbourne city in under 2 hours.
I would definitely pay $100 to reduce that. I think $100 helicopter flights is something I would do with friends just to get a drink across town occasionally. How novel!
Compare that to anywhere in Australia where you can be sure security won't take more than 10 minutes - you can arrive at the airport with 45 minutes before your flight and still be waiting 20 minutes after you check in and pass security before you board.
I'm not too big of a fan of the AirTram + Long Island Railway (LIRR) route, which ends up at one of the major train stations, has on outside terminal (not great in the winter and slow with a lot of things to remember to do), but is about half price of that.
The subway to JFK is cheaper, but most people who live in Manhattan and can afford to fly semi-regularly are going to spring for a taxi.