New York, NY (JFK) to Mexico City, Mexico (MEX). $224 round-trip, including all taxes. Travel any day of the week, all year round.
Flights are also available from Miami (MIA), Orlando (MCO), Washington D.C. (IAD), and Los Angeles (LAX) for the same price. Dates are easiest to find using the Orbitz flexible date search tool. Some flights require a long layover in Panama City, Panama (PTY) so keep that in mind while booking.