I'd hope for a 10-20 euros/dollars price, and perhaps the add-on coming with the new Wiimotes at no added cost.
In any case, engadget must be way off with this. Anyone who has a Wii and saw the E3 presentation must have noticed that this add-on was doing something which would be impossible with the current Wiimote.
My Mario Kart Wii friend code: 2707-1866-0957