Wii Sports Resort this spring. The bundle, which includes the new Wii MotionPlus adapter, will cost $49. This all came from a nameless Nintendo exec, apparently.
Seeing as how the original Wii Sports fetched the same price in Japan, and was eagerly lapped up there, we wouldn't be surprised if this was the case. Nintendo can certainly get away with charging as much, because, let's face it: most of us are going to be picking it up.