Is it common for tubes to outweigh the cost of the amp or preamp?

@samiamnot- Not exactly, "serial numbers"(though some use the term), as that would be specific to just one tube. Typically, date codes indicate a week, month, year and sometimes a location/plant, for every tube in that time frame’s run. Your seller can probably provide more data, for your specific tubes. https://www.tubemuseum.org/SearchResults.asp?Cat=24 6922s are dual triode(two separate circuit/channel) tubes, like most other, commonly used for audio, miniatures & octals. Basically, two tubes in one. The closer the measurements(hence, "gain"), between two separate, paired tubes, the closer your channel balance will be, when listening. Regarding tubes in a push/pull circuit; one wants the individual triodes balanced, to achieve the most accurate(least distorted) sine wave(signal) reproduction.

20 years ago I could buy NOS (real new old stock) Siemens & Halske Cca 6922's grey plates  for $40 each.  Bought a dozen for back stock and wound up selling 4 of them for $800 a year later and then 2 more for $500.  Now, any of them that moves the needle close to the good indicator is sold for NOS at $400 and more, much more some times, each. Real NOS is unobtainium.