Smart key dead questions
 

So for about half a year my smart key for my 06 RX8 has not been working. At first it seemed to be going through a battery a week until it finally stopped working all together. My question is since I only have this one key, I wont be able to buy one of ebay and program it myself right? Is my only option buying one from the dealer? If so is it only the key that needs to be replaced? (I've read that a sensor or something might need to also be replaced, not sure if this is true) Does anyone know how much the key costs at a dealer? I have read from $180-$500 on here. Thanks!