I didn't know that, but that doesn't surprise me. In any case, the Wii Motion Plus isn't going to be the same price, so I'd say it'll be $15 minimum, though I really expect $20. Anything higher is getting excessive for a controller accessory.
Though I think it would be a better idea to sell it as cheap as possible ($5? $10?) to get it into the hands of everyone, especially if they plan to use it in many games.