The cheapest way is:
Q10 bus JFK - Lefferts Boulevard $2.00 It goes from Terminal 4.
Subway line A to Chambers St $2.00 Walk to WTC station
Path WTC - Newark $1.50
Local bus Route 62 $1.35
Total $8.85
You may be able to get a free transfer with a Metrocard - you will need to check on the web.
I did this in April - prices may have changed since then. Standard disclaimer!
I only suggest this with limited baggage and no jet lag. Try a more expensive option if you are arriving on an intercontinental flight with heavy baggage.
Happy travelling!