What are the dates that you need?

The paid lots by Newark airport, off of Route 1 & 9 would be by far the cheapest and the best bet for that long of a time away. But if you don't want to go out to Newark airport, there are some parking garages at Journal Square that are fairly reasonable from what I remember. Some but not all of them take credit cards and they are right across the street from the Journal Square PATH station. Here's a website with more info on that:

http://www.thenewjournalsquare.com/js_parking_rates.asp

Where is a good (cheap, safe) for around 2 weeks?
Vacation time.