It depends on where you are flying out of in the U.S. to Mexico. I fly from Baltimore Washington (BWI) airport and many times can get $200 each way nonstop to Cancun on AirTran. Or similar fare less than $440 RT with one stop on US Air from DCA (Reagan) Airport in Northern VA to Cancun (got that fare for both 2012 Thanksgiving and Christmas trips).

I am flying on United to Cancun from Dulles (IAD) at end of March and flying bakc mid_april, and I got a fare of $502 total w/taxes.

So I think it depends on where you are flying out from.

Go to Bacalar and enjoy snorkeling in the Sian Ka'an Biosphere! I can assure you that you'll be amazed by the place! No other place like that in whole Mexico.