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I decided to save my gold balls from last generation for the next, and got lucky with my third shiny Magikarp.
I decided to record my coin gains from gold balls:
>Rank 22 / Generation 18
>Max Level 32 Individual Bonus - +60% coins
>Level 05 Bonus - 009 coins (prev gen)
>Level 05 Bonus - 009 coins
>Level 10 Bonus - 019 coins (prev gen)
>Level 10 Bonus - 019 coins
>Level 15 Bonus - 043 coins (prev gen)
>Level 15 Bonus - 043 coins
>Level 20 Bonus - 067 coins (prev gen)
>Level 20 Bonus - 067 coins
>Level 25 Bonus - 078 coins (prev gen)
>Level 30 Bonus - 158 coins (prev gen)
It seems like Gold Balls appreciate in value if you hold onto them anyway, the 'karp bonus is just the icing on top.