So you want to purchase an unlocked iPhone 4 in some foreign country. Sounds good, I did the same with mine - and I bought it from an Italian seller who bought it in France and sold it for 200 Euros more. Not a great story, but I needed it. Anyway, iFun put together an interesting chart showing the prices of the iPhone 4 around the world.
It turns out, Italian iPhones are the most expensive. Hong Kong ones are the cheapest. Check out the image below (click for full size). But don't buy one in Italy.