This has been a hobby project of mine. It's saved me a ton and thought the book reading community as a whole may get some use out of it. It gets deals from across some of the most common audiobook retailers and displays deals that match the price point and percent off you specify. There are no ads, no tracking, and the code is under an MIT license.
tbr-deal-finder works by checking for deals on exported GoodReads, exported TheStoryGraph, or custom CSV (details in repo).
It works with Audible, LibroFM, and Chirp. I'm hoping to add EBooks at some point in the future.
If you're interested here's feel free to check out my project where you can learn more about everything it supports and how to install.