@inna You really haven't mentioned a specific tube type you are looking for but the type really does influence cost. Tons of good not terribly expensive tubes in the 6DJ8, 12ax7, 6SN7, and various other categories mostly preamp/driver tubes. You can also pay a lot for these types if you want. NOS power tubes tend to be more $$ especially DHT types. So the answer as with most things, it depends.
How expensive are best NOS tubes ? And how difficult to find ?
Phono stages, preamps and power amps. Any non-Chinese adequate substitutes? I know that if you have Lamm, Vladimir has a few sets of Russian tubes in stock for each particular unit, and it is not really expensive to retube them. But if you have, say, VAC or CAT or Atma-Sphere and want best longest lasting and most reliable tubes, what then ? Don't ask me why not Chinese, I can't talk about it here except that I don't trust their quality control.