Ok, so imagine if someone cut a game controller in half then sold each half seperate for a ridiculous price? Oh wait, someone DID do that - Nintendo.
The Wiimote goes for $40 while the Nunchuk goes for $20. So together its $60 if you intend to buy extra or replacement controllers.
On its own the Nunchuk is completely useless. It has no function or power without being plugged into the Wiimote. It gives you access to the C and Z buttons as well as the joystick for easy maneuverability in various Wii games. Some games won't allow you to play them at all unless its attached while others might only require the Wiimote or are also function with the wired controllers like the Wii Classic Controller or the Gamecube controller.
Ok, let's look at the competitor brands. The PS3 DualShock 3 Wireless Controller goes for $55, however you can also just get the Logitech controller which is just as good or better for $35. The XBOX 360 wireless controller is $50. Since I don't trust any other third party brands currently making Wiimotes, I don't know of any cheaper alternatives for it.. making it the most expensive of the major next gen consoles.
What else does the Nunchuk function for? Well in boxing games you use it as your other hand to throw punches, block, move around. In a game like de Blob its needed to move around, change the camera, and target enemies.
So in the end I think Nintendo should just package both parts of the controller together and save on packaging and strongly consider lowering the price.