Prices out of EWR are ridiculous, and United can get away with it (and is) because they are the only non stop option to Cancun out of EWR, people will pay for non stop. The typical fare for the EWR-CUN route is now about $550, gone are the days of anything in the $400's. $300's (which I have also paid) are beyonddddddd history. I'm trying to get to Cabo in October, which is low season and the fare is almost $900. Disgusting.
Having said that, the fares may come down from what you're now seeing. I've seen some of those $550 fares come down into the $400's, rare but it does happen. March is peak high season, and if the plane is almost full to full, it's very very unlikely you'll see that fare drop. Maybe if you are flexible with your dates, you might have better luck. You might also want to sign up for Yapta, and tag the flights you want. If the fare drops, Yapta will e mail you. I see fares in Feb are down in the $500's - so that could happen for March too, it just depends on how full the flights are. And try not to fly on a weekend, that's going to bring the fare up too.
If you tell me what dates you were using, I can tell you how many seats are booked on the flights offered that day. Still not a sure indication that the price could drop if they're not heavily booked, but it might give you some hope if the flights are still somewhat open.