Some sort of guide to dispel some of the rarity myths might be a worthwhile endeavor even if it doesn't include prices.
Seems like a much more useful and doable idea to me, a la the way Atari stuff is ranked.
Prices change and fluctuate, and someone would have to keep up on that faithfully to make the price guide worth a damn, if anyone could ever agree on the values.
But a rarity ranking of the library once done is....done (maybe you add homebrews as released if you are so inclined). Unless case recyclers use up so much Power Golf it actually becomes rare!