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

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.

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.

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 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. 

That's what I would want to avoid - "stupid money". Now, if , as an example, $1500 pair tubes sound best and last me for 10 years that's not what I would call that.
Would you say, generally speaking, that if I listen for 500 hours a year, all tube equipment, the cost of tubes should not exceed $1000 per year ? It would still be $10k for ten years.