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

In most years no expenditures (needed or not) for tubes. I test all tubes periodically. I had some VR105’s go and some 6AX4’s go this year and they are inexpensive, even with marching. One Linlai 300B drifted way out ouf spec and I replaced the quad of the Linlai 300B‘s with a quad of Takatsuki 300B’s. That cost 3k and quite possibly the best tube roll ever experienced. Good from the get go and stunning after some hours used. A glowing report. 
 

This year $3200 spent. Last purchase was a spare quad of KT170’s three years ago

I bought a new matched pair (consecutive serial numbers) of McIntosh MC275 Gordon Gow Commemorative edition amps, the first re-release, in 1993. I used them configured as mono blocks to drive my Martin Logan Quests. Though the amps came with Richardson KT88’s made in LaFox, Illinois and Chinese 12AX7’s and 12AZ7’s, after about a month I replaced the Mac-branded tubes with Sovtek 6550’s and NOS Mullard 12AX7’s and Westinghouse 12AZ7’s. When I finally, and regrettably, sold the amps in 2012 because of a move to smaller digs, the set of tubes I installed still tested as new, with both amps still outputting 90WPC in stereo exceeding McIntosh’s specifications.  These amps were used daily, not just for music, but for TV, too.  So no tube replacement whatsoever in 19 years. Good thing I set aside the original Mac tubes.  I nearly doubled my money when I finally sold the amps. 

My Cayin CS-845A uses 3 different pairs of tubes, 845, 300B, 6SN7. I use it as my amp. It plays 3 hrs/day. I change the tubes out every 3 years (3000 hrs). Last set of tubes 2025 were Psvane Summit series, about $2000. My preamp LTA Microzotl level 2 has 2 pairs of tubes, 6SN7, 12AT7. It is easy on tubes. So ... a long winded answer is $660/yr. That's using Psvane's best tubes.

Perhaps $300 for 2 years. Tubes last a long time!

I had a tube go south on my power amp the day after my earlier response.  The replacement price was $100, but I ordered a full replacement set for one of my two amps (about $ 700) and figured while I was doing that I might as well get the 18 tubes for my preamp, another $700.  Keep in mind that these are only spares and the existing tubes have been in use for over 10 years.  In other words, I wouldn’t let the replacement cost keep the OP from purchasing tube equipment.