I bought some silver pendants with various stones in Costa Maya last year. Of course none of them had prices on them, and it was necessary to bargain for the pieces.
Imagine my surprise to see many similar items at Epcot in the Mexico pavilion, at much lower prices!
Since I do not cruise to the Caribbean to shop, I don't add in the cost of the cruise to the prices of the items I purchase. Shopping is just frosting on the cake, as far as I am concerned. Sometimes I don't spend a dime in a port!
I do buy David Yurman pieces in St. Thomas, saving about 15-20 percent on each piece, plus no sales tax. Since Yurman never goes on sale here, I consider purchasing it in the Caribbean a good deal.
As others have said, you have to know your prices.