Japan has been a major consumer of US labels for years (mainly in the mid 2000's) and due to China's emerging middle class, they have become a prominent consumer as well. From what I understand Amsterdam and Barcelona are also growing "streetwear" cities.
Even though streetwear has gained a significant amount of mainstream popularity, it will always be considered a niche luxury product as opposed to simply a fad. It's been around too long and continues to develop. But this isn't to say that certain brands don't have their "moments" where they are quickly gain insane popularity, then fall out of favor. That goes for anywhere in the world (via the internet).
As for import costs, I have no idea. But I'm sure that currency exchanges play a part in price differentiation, if there is any.