It may not be all that rare in the whole world (I think according to the Roland Vink site they made a little under 10 thousand of the AI version), I have seen very few for sale. In fact the only one I can recall is the one I bought from KEH many years ago.
Mine was a "bargain grade" one, and the first and only lens I've gotten from KEH that may have earned that rating. Though the glass was fine, it was engraved (LA Times) and a bit beat up, and was missing the tiny little setscrew in the mount that keeps it from over-rotating. Fortunately I found this out on a Nikon F, whose back comes off and allows removal without damage. If it had been a digital camera I'd have been in a mess.