Here's something I've been doing for a while that's really handy:
- View your wish list on Amazon
- Filter the list by items with price drops
- Sort by lowest price
I keep my Amazon wish list up-to-date even though it's more effort than just hitting "want to read" on Goodreads, and I keep a tab with this filtered wish list view open on my phone. Every couple of weeks I'll take a look and I usually find something new that's dropped to $4 or less.