You can buy new or used, but when you buy a used key make sure it has the white tag so the dealer can program it. A new/used remote key runs anywhere from $100-250.
One of the things I did was call around to local locksmiths, and found one with a laser key cutter. I had him duplicate my key for $40. The catch is that since it doesn't have the electronic head, if I ever try and start the car with it the car will immobilize. But is great to keep in my wallet or for when I go to the beach.
