Like I said you can find both for cheaper or more expensive. Since the key does nothing more than locate the flywheel on the crank while you tighten it down. You could have saved even more money by installing it with no key at all. Just line it up so it is half a key off.
You would have saved an aditonal 15 bucks. For a grand total of 35 dollars vs running a key and only saving 15.
I could see people running a key if the only plate that was around what the $100 Pro Design Plate, but its not, you can get a 35 dollar cast one. Heck you can get a nice billit plate from Chariot for 50 bucks.