Just to expand a bit on what's been said already, last time I looked at Mitsuwa (last year), whole wasabi rhizomes were $100 per pound. A bit steep perhaps but a $10 chunk will produce quite a lot of grated wasabi.

By the way, those reddish buds next to the wasabi are myoga, a type of wild ginger. I didn't know what to do with them at the time but after reading
this, I can't wait to try some.
I've also ordered the tubes of wasabi paste from Pacific Farms and was pretty happy. It's definitely not the same as fresh but it's infinitely better than any "wasabi" powder you'll find. Note that if you order the large tubes you get 3.5 times more for only $8 extra. Find a friend who likes wasabi!