That's right - strange as it may seem, it's the most recent one which is the rarest and most valuable; in case you ever come across it, it's "Tom Swift and the Galaxy Ghosts" - if you do see it, buy it for me, would you?
You're absolutely right about the reason; the series was being "wound down" by the publisher and the last book had a very small print run and most of them went to libraries. Even ex-Library copies fetch upwards of $500.