This is incorrect. NJ Transit's off-peak roundtrip tickets are valid between 4 pm and 7 pm for inbound travel to New York, Newark, and Hoboken from more distant points. The off-peak roundtrip tickets are not valid for outbound travel during this time period. So if the OP plans to take NJ Transit to EWR, buying an off-peak roundtrip to Elizabeth would work for him, and would be the cheapest way to take NJ Transit.

However, there is an even cheaper way to go, if the OP really has more time than money: From Grand Central, take a downtown 4 or 5 express subway to Fulton Street (12 minutes), walk one block west to the PATH terminal, take a PATH train to Newark (22 minutes) for $1.50, and then take the #62 bus from Penn Station Newark to Terminal A, B, or C at EWR (20-25 minutes) for $1.25. The subways run very frequently, the PATH every ten minutes, and the #62 bus every 10-15 minutes or so (schedule here: http://www.njtransit.com/pdf/bus/T0062.pdf). And if the OP decides to take the bus from CT to Port Authority, (s)he can take a downtown E train to the last stop (Chambers Street/WTC) and get the PATH train there.