What region are you in? I'm in EU (sweden) and it's in my store.
It looks like the original $10 release has been delisted: https://store.playstation.com/#!/en-ca/search/f=^farming%2Bsimulator|bucket~games%3Aplatform~vita|?smcid=sen-tw
As Takao said, the first (and cheaper) version of Farming Simulator has disappeared from both NA and EU Stores.
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