Newly released information from Nintendo now states that you can replace a busted Wii U Gamepad for a fee. Most likely the fee is the retail cost of the controller, as well as shipping. The supposed retail price ($180) was reported from Japan, but the actual price may be lower than that, considering Japan has a higher conversion rate.
Nintendo still confirms that individual Gamepads will not be available for sale outside of Japan for a while; however, you could get a replacement if you sent them in. If you are thinking of calling up Nintendo for a second controller, it is doubtful your plan will succeed. More likely, you will need to ship the controller to Nintendo to prove it is broken, and then pay the shipping for the return travel as well as purchasing the new controller. The fee could also add in a few extra costs but there are no details currently available.