Do you have a “price too high” wishlist where you buy your books?
Amazon in particular will let you create multiple wishlists, both for the public to view and for you to keep to your own little self.
Goodreads will let you shelve books in groups you get to name, so creating a list there works, too.
However, if you add a “You’re Kidding!” list on Amazon, and check it regularly, you will be able to slide down the list and see if any titles have been discounted, even temporarily.
Apps that watch for you
Apnoti
Apnoti watches Amazon for price drops.
CamelCamelCamel
Amazon and a few other retailers.
EDealInfo
Track an entire genre of products if you want.
eReaderiQ
Tracks Amazon books.
Happy hunting!