What about the magnetic key box for $5 dollars or less??? That's always worked fine. Also you can get a thing simular to what houses for sale have of the door knob. They make those safe boxes with a U shaped lock to lock to you car. A good place is where the safety chains hook up so you can still tow with your car.
Here is a pic of what I was thinking of and it less than half the price but still a lot more than a spare key.