My experience with duty-free stores is that independent of any currency conversion fees unless you live in a country where booze and "smoke" are particularly expensive, you'll find that they're generally much more expensive than "normal-tax" stores in that same country. This is particularly true for items such as electronics and clothing. Two reasons for this: 1) people often don't pay attention and buy stuff at the airport (possibly a collective, fake memory of what used to be true 50 and more years ago) and 2) the exorbitant prices airports throughout the world charge for renting the space!