Wow! That's really expensive! Doesn't the Wii Pro Controller work on it, too? That seems like a cheaper option for a traditional controller.
The Gamecube controller retails for $30 and the adapter is $20 and neither have been in stock in the last few months since Nintendo first announced them. This, unfortunately, is the only way to get the adapter at the moment without going to eBay. Folks have been paying anywhere between $40 and $70 for the adapter itself, so $50 for the bundle is probably good enough for folks.
I don't play Smash, but folks consider the Gamecube controller the best for the game.
Also, isn't the Wii Pro Controller $50 by itself? If folks already have GC controllers around, this seems like the best deal (if you pay retail for the Pro Controller).