Depending on how many days you park it can get expensive. I usually leave my car at home and catch a cab since sometimes, well often, they tell me to go out for a week, then it winds up being a month or two or three. Daily parking would add up way more at that rate.
Cheapest on airport parking at EWR is $18/day economy long term.
Parking - Airport Guide - Newark Liberty International Airport - Port Authority of New York & New Jersey
There are some off airport parking lots for $9 or $10 a day if you search online.