Where in Japan are you? I'm in Osaka and always looking for people to go riding with. I started snowboarding after I came over here so I don't really have anything to compare it to, but deep powder isn't uncommon at all, especially in Hokkaido. Snowjapan.com is a good site (the Now! reports and resort info more so than the forums). It seems like actually going snowboarding isn't that expensive compared to other places, but the mark-ups on gear are ridiculous. Look for deals on Yahoo Auctions. My favorite resorts are Takasu Snow Park in Gifu and Niseko in Hokkaido. I haven't found one I like in Nagano yet, but there'd definitely be a good one there too. I'm hoping to go to Nozawa Onsen, Asahidake, and Hakkoda (among others) this year.
If you're up for a trip to Niseko from 1/6 to 1/9, send me a PM! Right now it looks like I'm going to be going alone, and going alone is expensive.