I don't think an extra GamePad would sell at $100 especially since there will be so few games that use two. It needs to be priced as $80 to have a chance, but $70 or $60 would be better. The only way it could be $100 is if it sold with a game that has a two-GamePad multiplayer mode.
Preferably, the extra GamePad they sell should be smaller and more ergonomic, since the current GamePad is a monstrosity in all the wrong ways.