Just how much do you spend on tubes per year on average ?

A lot depends on type of tubes employed and how hard the tubes are driven.  My system happens to use tube types known for long life and my amps don’t push the output tubes at all.  This is VERY important as some of these tubes are quite expensive.

Well, I use Mullard EL34 XF2 power tubes. They are known to last for a long time but rarely longer than 5000 hours, as I understand, even if NOS and pushed gently. So, I think, in my case I should expect $500-$600 a year to lose on all tubes. My $450 monthly electric bill in New England makes me uncomfortable but the tube bill will not.

I no longer have anything that uses power tubes. I currently only use Dual Triode tubes (12au7, 6922, 7308 and…) and they last 9-15 thousand hours, so to answer your question, very little.

My wife grew up with her dad's system crippled by tube failures ruining her music. She bans them from our system. It's not about the money. She just really hates tubes.  

@inna running power tubes beyond 2000 hrs is very risky in my opinion. I had a power tube failure in Rogue ST100 amp and it took out several parts on that channel. Was repaired by Rogue. I sold the amp shortly after. 

With VAC the repairs will be costly. Up to you of course but I would swap tubes at 2000hrs and box them up for spares. 
 

I also had NOS small signal tubes go bad in preamps but they usually fail more gracefully than the power tubes. They became microphonic, were farting and making other weird noises. All tubes were purchased new from upscale audio or tube depot. Several failed in a course of 5-6 months. 

Highly dependent on how hard the circuit is driving the tubes. 
If you have a good supply of tubes you’ll be fine but I wouldn’t push the hours envelope.