We use Vista Parking. Without coupon they are $13/day. With coupon they are $11/day. Easy to get coupon. Just google airport parking and Newark airport.
We like Vista, because it has 24 hour security, patrolled by people like ex-police officers and it is gated. You pull up into the first parking spot, leave them your key (it is valet parking). They help you with your bags onto the shuttle and drop you off right at your terminal. When you come back, you call them and they come pick you up. They bring your car up while you pay.
Call us paranoid, but we wouldn't leave our car at any parking lot near Newark airport unless we knew there was good security. You may want to check if the place you're leaving your car has good security, or it may not matter to you. Either way, have a good trip!
