I routinely catch arowana and dorados. These sell for 10k. Saddled Bichirs sell for 4k, and I haven't found one. I've also experienced similar situations for other fish. Why is a cheaper fish so much rarer?
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Rarity does not always equal value. For instance, a common fish that is easily eaten will be more useful, and more valuable, than a relatively rare fish that can't be eaten. Unless we factor in other concerns, like extreme rarity and scientific value, in such cases as the coelacanth.
With insects, we mostly consider collector value. The big stag beetles go for lots of money due to their size and brilliant colors, along with the fact that they are relatively rare and easily preserved. Butterflies, meanwhile, are damaged very easily during capture/preparation, diminishing their value despite the rarity of some species and their vibrant colors.
So, it still is a factor of supply and demand. You're just making the mistake of assuming that if something is rare, it has a demand. This is not always the case.
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