More expensive, yes, but not THAT much more expensive. 3x as expensive is a gross exaggeration.
Here are some prices at two sites:
Newegg and
Prime PC. Today's exchange rate is about 119 yen to 1 dollar.
HITACHI Deskstar T7K250 (SATA)
11,885 yen ($99.87) Prime PC
$105.50 Newegg
CORSAIR ValueSelect 1GB 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) System Memory - Retail
12,838 yen ($107.88) Prime PC
$89.90 Newegg
AMD Athlon 64 3500+ Venice 1GHz FSB Socket 939 Processor Model ADA3500BPBOX - Retail
25,219 yen (211.92) Prime PC
$201.00 Newegg
I bought an enclosure a while back for about the same as I paid for one back home. However, I have found USB flash drives and digital camera memory cards to be a fair bit more expensive in Japan than in the US.
Shipping a box of computer parts via USPS will probably cost you around $100(insurance not included) unless you take the sea-mail option.
You could even get hit by customs, if there's a tariff for computer parts. I had to pay $20 when customs opened a box of contact lenses that I had shipped to me, as well as wait two weeks for the paperwork to be processed.
It all depends on what your friend is planning to buy, really. He might save money by buying in Japan, he might save $100 by having you ship, or he might create a hassle for both of you to save $10.
Personally, I'd price check the parts in both countries before making a decision.
