Parking on-site in EWR's long-term economy lot will cost $4 per hour and up to $18 for the first 24 hours. After that, charges are incurred in eight-hour increments ($6 per eight hours) with a daily maximum of $18. There are also cheaper long-term parking options as low as $8.00 per day you can find using a nearby offsite lot.
Additional information for EWR parking rates for short term and extended stay parking can be found at the official web site for Newark International Airport.