Retail price alone isn't enough to judge where it's cheapest. You have to find out whether the location charges sales tax, VAT, etc. The retail price for a low-end iPad is $499 in the U.S., for instance, but whether you pay tax and how much you'll pay will vary, depending on where and how you buy in the U.S. Buying online can save you taxes, for instance, if the company has no operations in your state. But then you might pay for shipping, depending on the vendor.
From my experience, Hong Kong retail is cheaper for iStuff, because of the exchange rate and no tax. But it's not that much cheaper. And I'm talking about legit retail, not gray marketed or stuff that fell off a truck. YMMV.