There are free ervices which do those amazon price checks for you and email you when triggers you set are met
I don't know for sure if they work for Kindle books like they do for physical goods though. I think you can even specify a country.
And there used to be a specific one that did books and only books. Sorry I don't have the name of that to hand and it may not still be operational, but I used to use it to tell me of any price drops for specific authors..
Hopefully someone with a better memory than mine will name drop... Or Google will know...
Update
Kindle price drop tracker
https://uk.ereaderiq.com/track/drops/
Thanks google