Two Agatha Christie titles have dropped to $1.99 in the US.
A Caribbean Mystery, Miss Marple #9
Info and links in
this previous post
Cards on the Table, Hercule Poirot #15
Kindle US:
https://www.amazon.com/dp/B000FC10ZC
Kindle US/Smile:
https://smile.amazon.com/dp/B000FC10ZC
Kobo US:
https://www.kobo.com/us/en/ebook/cards-on-the-table
Also note that
The Murder of Roger Ackroyd (Hercule Poirot #4) entered the public domain in the US on Jan 1 of this year (pubbed in 1926, see CrimeReads article
here), so you can now find it at some public domain sites, like
Internet Archive, although many, like
Project Gutenberg and
Faded Page, don't seem to have any version uploaded yet.