My spare key cost me about 30 bucks (from the stealer) and if my memory servers me correctly, a replacement remote control key would cost around 125 bucks.
Also, IMO...if your not sure when the last time the battery was replaced in your original key, then I would recommend that you do it sometime soon because you don't want to be stuck out in the rain trying to manually unlock your car or, worse yet, unable to disable your alarm system

Can't recall, but it may also effect your ability to start your car??? Someone else may be able to verify that statement...anybody?
And, when and if you do replace your battery read your manual because there is more to it then throw the battery in and go.
And BTW, welcome to the forum! So, how many miles are on your car?