As good friends it will cost 1M. If you become BF it will go down to 40k.
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Jeez
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1 million for legends and shinies not registered in the recipient's Pokedex. IVs get rerolled and can go below 10/10/10 for legends.
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People are too eager to trade. You need to wait until you and your friend hit Best Friend Level to both reduce the stardust cost and ensure better IVs on the mon post trade.
Wait. Wait. Wait.
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I would bet both Rayquaza and Entei become available somewhat soon, before the friend status progresses enough. Entei thru field research and Rayquaza's shiny hunt. Try to be available to raid more Rayquazas and make sure you get a field research task completed everyday