How expensive are best NOS tubes ? And how difficult to find ?

When I had an Octave V70se and later the V110 I tried a lot of different input - splitter  and power tubes. I always preferred the sound of vintage/out of production tubes. With some the difference in sound was huge and in others not so much. As far as NOS who really knows if they really are NOS or used.

That said I started with Dyn C1 sig speakers. My favorite tube was the original TS 6550 black plate followed closely by the solid grey plate. With those the little Mullards were a great compliment. Then I got the V110 and hated the Russian KT120's. The old TS were still my favorite. I also tried some British Gec KT88's. Didn't really care for those but not bad. Jim McShane told me about Ei KT90's but he wouldn't sell me any. When I first found some here on the Agon I bought them. These IMO were very good and at the time pretty reasonable. Then I upgraded the Dyn C1's to Raidho D1's. Now I didn't care for the TS 6550 or the little Mullards. KT120's sounded a bit better but still not good. The Gec KT88's and Ei KT90's were the tubes that sounded best with the D1's.

My point is all tubes have a sound signature that will vary with manufacturers (not always labels for they may be only one manufacturer that makes many labels).

Well for me the TS 6550 bp has the same glorious mids as the EL34's but adds a hair of warmth and has the lows EL34's don't have besides more power. I felt the EAT KT88 has the most linear sound from top to bottom along with the original KR Audio KT88's. Just saw an ad for new KR KT88's which may be back in production. Ei KT90's are somewhat linear with a bit more in the lows but had the best sound stage of them all. The original Gec KT88's are also quite linear.I didn't use them much since I preferred the Ei KT90.
I'm sure transistors also have a sound signature too. To me Pass labs or Dartzeel is a much warmer sound than SimAudio or Soulution as a couple of examples.
Going back to speakers I preferred a warmer sounding amp when I had the Dyn C1's and a more linear sound with Raidho. But there is also the beauty of a tube amp. If you want a different sound signature you can swap out some tubes and not have to purchase a new amp.