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. 

Inna  I can't see your cost being that high. But who knows if I sat down and tallied mine up I may crap my pants :-) 
But solid-state is not an option for me, the things I value such as dense harmonics, presence, sounds staging is all what tube gear excels at.  Basically I find tube gear to excel at natural musicality. And then there's the cool factor!

 And I have had good luck with Chinese power tubes as well