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

Cheap ---- with nice light bulbs won’t be a diamond when dressed up, just better looking. But a quality amp and these exotic tubes you are getting raped on...no, it is not common. It’s frickin stupid! Plus what happens when you toast one? You’ll never find another pair. Then you’ll be crying the blues once again!

The Siemens gray shield CCa tubes are considered by many to be one of the best 6922/E88CC ever made. That doesn’t mean that you’ll like the sound they make in your headphone amp or that you’ll hear how good they can sound in a top tier component.

I’d say hang on to them if you plan to own a top tier component one day and experiment with good but not top tier tubes in your headphone amp. This will tell you how good your headphone amp is at conveying the sound of different tubes and how good you are at hearing the differences between tubes without spending big $. If after experimenting a while you think your headphone amp and you are up to the task of appreciating the best, roll them in and enjoy!

My amp cost around $900 all tubes included. A replacement set of the power tubes is around $360. IMHO the power tubes are insanely overpriced garbage. The preamp tubes that came with it cost about $9 each to replace with the exact same ones. I've made tremendous progress by getting cheaper power tubes and vastly more expensive pre's. If you have the money, go for it, you only live once.
I do believe that the law of diminishing returns applies in all situations, yours included. The $700 tubes are likely only marginally better than some that can be had for 1/4 the price. One example being telefunken 12ax7's for $500 each vs a similar vintage rca or mullard that can be purchased for about 10% as much. IME the differences are minor.