An interesting question... has it come up here before, though?
I used to think of a lot of my imported jazz CD's as being rare and hard to replace, but a lot of them have seen repeated reissue (TOCJ's, Columbia stuff out of Europe, MPS titles, etc etc). Maybe the Lim. Ed. OJC's are the most likely to increase in value?! Anyway, I'll have to see if anything pops into my head in terms of jazz discs...
On the other hand, I've got a lot of brazilian CD's that would be difficult or impossible to replace now. Ironically, what I think of as the "Kind Of Blue" of modern Brazilian music ("The Legendary Joao Gilberto", released domestically on World Pacific) has been oop for over a decade due to a conflict between J.G. and EMI. Imagine if Miles had kept KOB oop...
Of course, the value of a rare CD will also depend upon the demand for it, and I'll admit that some of my rarest discs (jazz or brazilian) will never be in great demand. So, the degree to which we cherish a disc may have nothing to do with monetary value. Now, if you'll excuse me, I think I'll go listen to that Louis Stewart/Bill Charlap disc I got from Ireland...