Why do Microphone tubes cost so much?

BAD TUBES 2011

Sometimes people wonder, when they send vintage mics in for repair, or even new production out of warranty, why tubes can cost so much. Sadly, the days of $1 tubes are over for most of us. And there is no new manufacture of useable mic tubes. Fortunately, there are still tubes available. But they must be tested, burned in, then tested again. That's after they are found. In short, there is generally a high rejection ratio for microphone tubes, due to the high impedance involved, and the sensitivity to amplify even the slightest imperfection of vacuum or molecule of gas. I've even had known, reliable preamp and guitar tube tester guys send me tubes that were "super quiet" for guitar amp or hi-fi preamp use, that were unacceptable for pro mic use. See the above photo for last year's bad tube collection, the middle yellow bin being the most expensive: the EF86 for use in U67's. With a high rejection ratio, and an average price over $25 (off the top of my head) you're looking at one expensive tube. Of course, rejection levels vary with production lot, but even then, it's no guarantee. And then there's all that labor that goes into the selection process, including the rigs you'll have to build yourself.
As an example, we recently replaced a submini tube (which can be found untested for under $100ea) in a Neumann M147, which cost  over $300. Cheaper for us to buy from Neumann USA than for us to find 30, 100 or 1000 pc and the test test them, especially since they are wire leads and not pins, therefore requiring Greater labor just to test.
Note to the wiseass who wants to comment on their "long streak of dumb luck" in finding what they call acceptably quiet mic tubes, please don't bother, I can probably prove your tubes are noisy.