Cos I brought a DVD box set from Japan that cost me $270.
Why is it so expensive?
1. High prices to begin with.
2. The killer yen which makes it even worse.
3. Competition (or the lack thereof).
You didn't say what item exactly, but transport and selling of items in Japan with high fuel costs/taxes and high rents drives things way up. So does Japan's insanely byzantine and bloated distribution system. And the fact that you didn't pay less somewhere else shows that you are willing to pay through your nose for what you want, like many others.