Last May when I flew Southwest to Cancún it was $277.96 per person round-trip from Manchester, NH.
Today, the best I can find would be $592.25 with that sale at the end of this month. I do miss those introductory fares!
As I'm sure you do, I constantly follow fares from my area to Cozumel. Unexpectedly, every time I've priced fares over the past year, Southwest has been more expensive than other airlines. Since they only go to Cancún and not Cozumel and since I can't get direct flights even to Cancún, they're just not a good deal for me at the moment.