Now that the iPhone 4 is available around most of the world, iFun designed this interesting little infographic that compares the various prices of the handset around the globe. Note that all of these prices are in Euros, and that they all reflect the out-of-contract price -- because the US iPhones are locked to the AT&T network currently, our country doesn't appear on this list.
But if you want to buy an unlocked iPhone elsewhere in the world, you should probably go with Hong Kong, where an unlocked 16gb phone will set you back US $653. Italy sells the costliest iPhones, with the price reaching up over US$1000 there for an unlocked phone.
This whole thing is really just more of a fun comparison rather than a really solid analysis -- most people who want an iPhone will probably buy it in the country where they live, especially if they need a certain contract. But it is interesting to see just where Apple has determined it can go higher on the price, either because of certain tariffs or shipping that must be paid, or just because the demographic calls for it.