The game market in terms of valuation has been hijacked by these "graders" & speculators, & it's gross-inflated-pricing is further perpetuated by folks expecting similar payouts. There is genuine "rarity," and I guess what I could call "artificial rarity."

Genuinely rare games are usually those printed at the end of a console's life-span that received only one mass printing. Another example of genuine rarity would be contest or competition cartridges, unreleased prototypes, or literally anything they only made a few hundred copies of.

Silent Hill 2 is not genuinely rare. Earthbound is not genuinely rare. Kirby Extra Epic Yarn for the 3DS is not genuinely rare. These games are just "desirable." Who defines what is desirable? Who makes cartridges or discs that were mass-produced hundreds if not thousands of dollars? Hucksters do. And every day folks that buy into it.

To answer your question: there is no way to determine the value of these things. When literally all it takes is a YouTube video or a Wata plastic trophy sale to inflate them, they are all virtually worthless and priceless simultaneously. So, if you ever want to screw with the market, sell your games at a lower average cost, and your one sale with affect how much people will be willing to pay for anything. It's all artificial.