According to the Adopt Me! Wiki, a Neon pet is made by fusing 4 fully grown pets of the same kind at the Neon Cave, and a Mega Neon by fusing 4 fully grown Neons. That means one Mega Neon costs 16 normal pets. PetTradeCalc therefore multiplies Rarity Index by 4 for Neon and 16 for Mega Neon. Market prices for Neons can be higher or lower than this, depending on demand.
Source: Adopt Me! Wiki – Neon Pets.
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