All the Carl Zeiss Jena's are collector items. Becoming more desired each year as their numbers of available lens dwindles. Some focal lengths more than others. Some remember owning one in the past or a relative or acquaintance owning and using one, so they now want one for memories sake. Others have read that they are very good and want one for that reason. And others want to collect them for their rarity.
Either way their value will increase. A while back someone wanted me to modify one for a K-mount and I told them to reconsider, as it is a collectors item and the value will only go up in the future. I still hold that belief. Sort of the same way with the Vivitar Series 1 lenses. There are better new lens out there as far as IQ and AF goes, but if you have ever used one you know there is a feel to them ( heft and smoothness of function) That just screams quality. It is a feel that the new mainly plastic lens do not deliver. Some of them Like my Vivitar Series 1 105mm f/2.5 Macro lens are better than the new offerings. It is said a lot of the Jena's are that way as well, delivering as sharp a photo as any high price new tech lens.