Nikon batteries are chipped, meaning they will feed information to your camera on its real state of charge and its probable lifespan. Third party batteries are NOT chipped. There's a reason they cost so little. No one likes to pay the Nikon prices, naturally, but in this case, you get what you pay for.
I can get a battery for my D40 for $30 (3rd party). The Nikon battery will cost me $90.