You can actually get the bus to EWR directly from Grand Central, rather than going to Port Authority (the bus starts at GCT, then stops at Port Authority on its way to the tunnel). That's probably the easiest way.

The absolute cheapest way, on the other hand, is to take the MNR into Grand Central, then take a couple subways to the PATH train to the NJT 62 bus.

But I'd take the bus from GCT to save the multiple transfers.

See our guide to EWR transportation at:
http://flyerguide.com/wiki/index.php...sportation_FAQ