How low do you need to go? If you can take an extra travel day on each end try Spirit Airlines and go via Cancun.I put in early December dates from ATL and got $282 r/t. Spirit often gives coupons for $25 to $35 off - you should be able to get fares around $250. Then you can go overland by bus to Chetumal - overnight there and take the boat right to San Pedro the next day. For the return I stay on Playa del Carmen. I do the trip from Florida when the AA rates are too high. Costs me about $450 total as a single traveler. For two people it would cost less per person as you save on the hotel.