I think some of the people in here are confused....everyone saying $10 or less are most likely talking about the wii motion plus adapter that connects to the standard wii remote, not the wii remote plus(which is the wii remote with the built in motion plus sensors). The Wii remote plus goes for around $30 to $40 on average.
The Wii Motion Plus adapter is sold at all local retailers for around $20 which comes with the WM+ adapter and the long rubber sleeve to house both the motion plus adapter and the standard wii remote.