I paid 1200 US for a matched Quad a couple years ago, dated from the late ’70s. And that was a "buddy price." The earlier the better in terms of date of manufacture. I knew the source of these and knew they were not pulls, i.e. used but measured NOS. The came from Victor at BAT but went through a couple of different hands- and I bought mine from somebody I trust.
They are probably worth at least that if not more. Depending on how many you have, whether they are matched, etc. Know that these tubes are counterfeited and there is a page on the internet that describes what to look for-not suggesting yours are bogus, just that you should not necessarily go by E-Bay prices or whatever. Not an easy tube to find particularly the early production. And in my application, they definitely sound different than current production Sovtek or EH.