Perhaps some tweaking might be in order to the odds. After a little bit of testing, the numbers seem to be pretty in favor of a few of the key and very little for a few others. Considering cost/effort to make and the slimness on the return (randomness and the fact you are sacrificing a key), a touch more in favor of getting a rare might be in order. (Not even 100% sure a couple of the rarer keys are possible to get.)