Interesting concept, but I don't subscribe to the notion that keys are defined strictly by value. Value is too fluid.
Is Action 1 any more of a "key" now than it was 40 years ago when it was within the financial means of an average person? (Sure it was relatively expensive then, but compared to average income, much more affordable.)
Because a previously undistinguished book features a character that appears in a movie and suddenly spikes in value, is it really a "key"?
I see "key" as being more of a result of consistent demand over time. Not sure how you'd measure that though. That's above my pay grade.