The specific events vary. I couldn't remember specifics, so I checked reddit:
This one was a flat 25% rebate on all Lapis purchases for Prime day. It was advertised as 25% off, but you had to have enough to cover the non-discounted cost, so it worked as a rebate. There was no limit AFAIK. The rebate was instant as well.
This one was for new Amazon app store customers only, but it was 35% off of the 10,000 coin bundle, so $65 for $100 more or less. There have been many of these sorts of events, though this was the highest that I recall. I've seen others for less.
This one coincided with the above deal. The $50 Lapis bundle had a flat -25% price to it, bringing it down to $37.50. This specific one was bugged and Amazon hadn't approved it yet even though the in game news advertised it and the in game cost showed $37.50, but on checkout, it showed $50. For me personally, this is when Amazon refunded purchases and gave the coins anyways, and Gumi followed up 2~ months later and gave me an additional 18k Lapis.
That's all I'm able to track down. I feel like there was one more event shortly after the Amazon version came out, but it may have been EU only or something and I'm just misremembering. All of these were also back when coins were regularly 25% off for the $500 bundle. It seems that's a rarity anymore.
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