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

 I bought a pair of NOS 845 RCA's and directly compared them against shuguang metal plate C.  I couldn't tell the difference so promptly sold the RCAs.  I got about $1500 for the pair.  I really can't think of one tube type that you need to go NOS for the absolute best sound.  There are modern production variants that sounds just as good in almost every case I can think of. However often times those modern variants cost just as much. 
 Also I find people that don't own or haven't owned tube gear have this idea in their head that they need to replace Tubes every two years or something. That's not the case at all. In my experience tubes last 5 to 10,000 hours. If you do the math on how much you listen per year you'll find that you rarely need to replace Tubes. 
 Don't give me wrong I do run some NOS in spots where it's not stupid money. Such as 7025or 12AX7 or 6SN7.  And often times it's not the most expensive holy grill tube that will sound the best in your piece in that spot. You need to try different ones and go with your ears. 

I have NOS Amperex7308 white label. But the others are modern - Takatsuki 300b and Sovtek 3H30. In the past I tried NOS 2a3 and 45 tubes, but was not that impressed. 

I agree with the above comments saying it depends on the specific tube and the application.  There are some very rare and high priced tubes, but overall there is no problem finding great sounding tubes.  It's been my experience that price doesn't fully equate to performance with NOS.  A Bugle Boy or Mullard may be at the apex, but a Brimar, Mazda or even RCA at a quarter of the price can be excellent tubes.

IMO, unless you are buying on Ebay or scavenging electronics at estate sales, you don't need a tube tester.  Getting an accurately functioning testers and knowing how to properly use it is a non-trivial task.  A good vendor can do all that work for you.  I'll plug Brent Jesse, but he's not alone.

Not all NOS tubes are expensive. And IME, with the few tube pieces I have, I’ve found that high priced NOS tubes don’t necessarily equate to best performance. As usual with audio gear: it depends. For example, I had an expensive pair of NOS tubes in my preamp which sounded great. I replaced my power amp and ended up having to replace my expensive NOS preamp tubes with inexpensive NOS tubes because they sounded far better (to me) with my new power amp. You simply cannot predict what will work best in your setup until you try it, so you cannot predict for certain that your future NOS tubes need to be expensive. Just my $.02.

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/

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.

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.

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.

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.

Call Andy @ Vintage Tube Services.

I think, Ypsilon too has in stock NOS Mullards that Dimitris uses.

Here you go.
http://www.audiotubes.com/