It is simple supply and demand, as things get more scarce and as the amount of people wanting them increases so does the prices, the Telefunken 803s is a perfect example of this, nice sounding tube no doubt but not worth 7-8 times more than an all ready overpriced 12AX7 IMHO but some want the best even if it is only by 2% and are willing to pay for it.
Status plays into this also being able to brag that you have a blah blah blah to your peers helps drive up the prices, and I suppose if you want the absolute best there is, money really isn't a concern but ownership is.
I know that after listening to a Mullard 10M Master series 12AU7 and 12AT7 in my Fisher 80AZ that I would pay hundreds if needed because they really do sound that good to me, The Telefunken 801s and 802s combo sounded pretty nice also but not like the Mullards did, others of course would prefer the sound of the Telefunken's and pay whatever they had to also.
Face it their not going down in value probally ever or until something better is made.
REGARDS JIM