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. 

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.

the expense is mainly in picking thru 1,000 tubes to match 10 very closely

there is a video on how Audio Research turns a cheap tube into a spendy hand-picked tub - same for caps, etc.

I have a very simple solution: I buy ARC tubes for my ARC gear (!)

I let their engineers do the jobs they were assigned to do.

Then I pay them.

That’s exactly right, randy. Finding the cream of the crop will cost the consumer.
I have a very revealing single-gain stage preamp and needed to return some microphonic 6SN7 tubes to Vintage Tube Services. Andy replaced them and told me that he needed to test 60 tubes to find a pair that he considered high quality, (and they fetched a premium price).

I find the price well worth it for my current CD Player and previous preamps, as the tubes can last a long time, not so sure about some high power tube amps...as for the engineers picking tubes, while I trust them, they are limited to tubes available in very large quantities, not NOS tubes...