If you order the 16GB flash drive directly from Nintendo, it will set you back ¥3,150 or US$40. For comparison's sake, you can get 16GB memory via Amazon Japan for about ¥1,000 or $13 (Sony 16GB drives are around ¥2,780 or $36).
For gamers who are shopping around, there's also a Dragon Quest X branded flash drive from Hori that costs around US$25. It comes with an in-game shirt.
Nintendo doesn't recommend players using this drive with anything but their Wii (probably due to piracy fears). Exclusivity doesn't come cheap and neither does Nintendo branding.
Wii USBメモリー [RVL-035] [Nintendo]