I would not even bother to look in Grand Cayman. It is more expensive than U.S. Even before our recent problems with value of the dollar, US dollar was only worth 80 cents there. Also, even if the exchange had been even their prices were and are high.
Not sure about Cozumel. Lots of good leather but Mexican made. Don't really know about designer stuff, as I refuse to buy it. But, better to try there than G.Cayman.