The difference is the WRP is a brand new wii remote that has Wii Motion Plus built into it. You don't need the accessory to have the wii motion plus ability.
Seeing as how almost every single brand new Wii game costs $49.99, I think its safe to assume that the game itself will cost that much. Wii Motion Plus and Wii Remote plus are a different case, however. If they release the game with the only buying option as the game bundled with Wii Motion...