Never.

Have you noticed how "rare" certain Murray or Huffy products are? It doesn't make them valuable; nobody wants them.

On the flip side, Schwinn Paramounts are the antithesis of "rare" - they're literally "coming out of both ears" in the C&V world - in all forms. It is easier to find a 1930's Paramount track bike then any other track bike of the era. While Paramounts are, therefore, quite plentiful; they are outrageously "valuable" - more accurately, overpriced - despite this.

Same applies to PX-10's, which are probably even more common.

-Kurt