Depends if you're going just one time or every day.
I would hop a train or bus from Elizabeth to Newark Penn and then just take the PATH from Newark to WTC. That would be cheapest.
You could drive to any of the PATH stations and park also. they built a new parking garage at Harrison but I don't know how much the parking is. PATH is $1.75 each way.
You could also drive to one of the NY Waterway ferry terminals, park and catch the ferry. That might be the most expensive way.
Check here for details
Pier 11 / Wall St.