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

This is not about retubing what I have now, I don't have tube equipment, this is about possible future upgrades. I suppose, best tubes is a significant part of the cost, and I am talking high level components. Not interested in mid-level something tube equipment.
And NOS tubes are old stock, prices must be going up and availability down. Say, $15k - $20k used tube equipment could cost double that within ten years of ownership, or what ? Yes yes, it depends on particular equipment and tubes, hours of play and stand by, pure luck and maybe other things. Still, should cost a lot for an all tube system even if not the double.

Not every stage in a preamp or amp needs NOS tubes. There are phase splitters and cathode-followers which have less sonic affect on the circuit. New production tubes are cheap and also sound good as power tubes. There are some really good quality Russian tubes now, and I believe they will continue to get better sonically.

Some of the high quality Chinese tubes cost as much as some NOS. If given the choice, I would buy NOS from a trusted dealer.

It's true that as availability of NOS tubes goes down, prices do go up. And the supply of true "New In Box" old stock is diminishing, but there is still a great supply of NOS tubes that are removed from electronic gear from military, medical, industrial sources. Although they are not really new, they have either minimal or zero hours of use, and will test as new.
That's why anybody who's into high-end tube gear needs to own a tube tester to check their purchases and their personal stash.

Your question is too general to give a specific answer other than to say that price is a function of supply and demand.  Just look at vintage Western Electric 300B, United 845 as examples.

In the link on Brent's site, click a tube that interests you and you will see his detailed descriptions on the history, country of origin and sound characteristics of all the different tubes used in the industry today. It is very useful info and his offerings are top notch.
http://www.audiotubes.com/