Almost all of the Rabbi Small books by Harry Kemelman have dropped to between $1.99 and $2.99 at Kindle US, and a few at Kobo US. This also includes Kemelman's Nicky Welt short story collection, and also Conversations with Rabbi Small, which (to me, at least) is not a mystery, but is integrated with the Rabbi Small series, and (IMO) well worth reading.
Kindle US:
https://www.amazon.com/gp/search/?se...price-asc-rank
Kobo US:
https://www.kobo.com/us/en/search?Qu...cmaxprice=3.99