But HPN will be a very cheap addon to EWR tix and an interesting anomaly will be that a Y fare from the north (say Montreal) will probably be less expensive to HPN than to EWR due to the shorter mileage between origin and destination.
In the days before the computers, we always bought our tix to the south from PCT (when Princeton Airlines was extant), got the cheaper fare, traveled to and from EWR, and used the coupons to commute on days when somebody else needed the car, walking across the street from the airport to our office.