I remember just only a year ago, Red Version for example (unboxed) used to be found at $40 on average, but now average price is only $25.
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I think the market is over saturated with them and the hype kind if died down to cause the price drop. The only ones with prices that haven't dropped are the brand new and sealed ones from the older versions those are still kind of high.
Prob pokemon go brought all the "casuals" back to the world of pokemon so maybe more people are putting the old games up for sale I don't know
Ps i used the term casuals for comedic relief...