"For the absolute cheapskate, JFK is potentially the cheapest if one takes the subway and the Q10 bus because it's $2.75."

Yes, that's true. But Manhattan to EWR can be had for $4.35 (less if you're 62 or older) by taking the PATH train to Newark Penn Station, and then the #62 bus (frequent service) to your EWR terminal. If you start from lower Manhattan, the PATH train from the old World Trade Center station to Newark Penn Station is a 22-minute direct ride; if you take PATH from other Manhattan stations, an across-the-platform transfer at Journal Square would be required.