GoNintendo reports that the latest issue of Nintendo Power states the manufacturer's suggested retail price for the Nintendo Wii Wheel is $10. It appears that some retailers aren't taking the suggestion to heart, as Gamestop, Amazon and some other stores are selling the glorified plastic shell for $15 separetly from the game.
Best Buy seems to currently be one of the only retailers we could find that had the product listed and selling for $10 (but we're sure that the Joystiq army of internet detectives can find more). We've contacted Nintendo for confirmation on what the actual MSRP of the Nintendo Wii Wheel is and will update as soon as we receive an answer.