I was wondering about the sizeable price discrepancy myself when I saw it listed on HobbyLink Japan:
http://www.hlj.com/product/KBYMK-03
That's quite a big price gap there.
The only thing that I can come up with is that it's a distribution matter. I'd imagine more of these will be released in the U.S. than in Japan.
If the U.S. distributor were different than the Japanese distributor, I'd wonder if there was going to be a difference in materials, like the difference between Hasbro's Transformers (plastic) and Takara's (die-cast metal). It seems like Kotobukiya's the distributor in both countries, though.
Anyway, a difference in the release numbers for either country is my best guess.