The "SouthWest" effect is no longer valid, where when SouthWest comes into a new market/city, they usually drive down fares from other airlines from the increased competition. Today, Southwest is just as expensive as other airlines.
Continental, Airtrans, FunJet, AeroMexico, etc..., used to fly Houston to Cozumel & Cancun for $300 - $400, & much cheaper with vacation packages.
Today, since United took over Continental, their monopoly raised prices to $550.
But hopefully, Southwest will increase competition (hope other airlines come back also) & bring prices back to reasonable levels, which I think around $300 - $350 is reasonable....
My take on the situation is that United/Continental and Delta (and perhaps American, too) have conceded budget Caribbean routes to Southwest and its former AirTran routes and no longer try to compete. Delta seems to concentrate on its business travelers these days. Delta's prices for nonstops from its hub here in Atlanta to Caribbean destinations are insane: over $700 for CZM and over $900 for BON.