I got the price-drop alert from Keepa. Here's the link:
You can track any Kindle book using Keepa (free). You'll get an alert when the price drops.
Here's how I use it:
When the prices drop on books like these, I buy HUNDREDS of them. You can do this on desktop (doesn't work on mobile) where you "Buy for Others" and get Kindle redemption links.
Then, use these as lead magnets and give them to people when they join your email list.
I did a whole video about this here: