Nintendo will not be selling the Wii U gamepad separately at launch in North America. Today the company confirmed that none of the launch titles for the new console will make use of a second tablet controller, so right now it doesn’t look like you could use it even if you could afford a second tablet.
Nintendo announced Japanese pricing for additional Wii U Gamepads, and they were listed at around $180. Nintendo did confirm that the Wii U will eventually support dual GamePad support, but since no games are taking advantage of the feature it will be unnecessary to own one. That is unless you accidentally break the new controller.
The Nintendo Wii U will also utilize a variety of new and existing Nintendo controllers. The Wii U Pro controller, which looks very similar to what you would find on the Xbox 360 or PlayStation 3 will be sold seperately, and will cost $49.99. The console will also use existing Wii controllers for games that have multiplayer functionality.