check out this
page....i would just double check any information you get to make sure it is accurate...but you should be able to find fares and estimated travel times
once you do the math, and IF you have time to see the city, i would suggest doing things allow you to come and go as you please (i.e. not a broadway show)....check out central park, ground zero (former twin tower site), empire state building, shop on 5th ave, go to the top of the rock, stop by a museum, get some good food (china town, little italy or any of the other ethnic neighborhoods), see the seaport or many other things....even just walk around or find a bar and people watch
you should do a google search for any special events that might be happening that day
keep in mind - you can't do all those things since they are located in different parts of the city....look at a map, pick what you like, and leave time for delays and traffic....enjoy!
also – where will you keep your luggage during all of this?