At the risk of sounding pretensious, I think I may have a bit of objectivity to share based on a project i just completed that I started during the first year of the pandemic.
First, a word of explaination. I am an 83 year audiophile who has rarely bought any gear new at full list price. This goes back to when I was in my thirties, after my very first system. Currently I have three full systems (bedroom, den, and living area) and two back porch rooms where used audio gear coexists with excess photographic gear. Currently this amounts to 64 pieces of audio gear (not counting things like record pucks, etc. (now you know why I can't afford a Porsche :-) ).
What I started during the shutdown was a list of all my gear, and then using my collection of old catalogs finding the year (or close to it) of introduction and the introductory price. I then went through the A'gon BlueBook to list the average private sale price for each piece. If the BlueBook didn't suffice or didn't seem right I often cross-checked with eBay completed sales. And sometimes I checked AudioMart for seemingly completed sales.
Finally, and most recently, I ranked the items by date of sale and used the latest CPI calculator (March 2023) to project the current introductory value of each piece. I then calculated the percentage of those introductory prices (original and inflation-adjusted) for each item. Finally, I took the average value "around" each five year mark and charted the results. Here is a summary of the results (sorry I can't show you the chart:)
Years After Intro Used vs Original Used vs Infl-Adjstd
5 118% 53%
10 85% 39%
15 50% 20%
20 40% 15%
25 50% 18%
30 40% 12%
35 61% 18%
40 57% 13%
45 91% 17%
50 61% 9%
55 62% 8%
Versus the original price, used prices dropped to roughly 50% of new after 15 years, and to only 20% of an inflation-adjusted "new" price. But after that used prices versus new stayed at the level for another 15 years and then began to rise. Prices against inflation-adjusted :"new" prices stayed pretty constant from that point on at 15-20% until the items were 45 years old or so.
Now, obviously a lot of other factors can affect individual prices, but this seems to answer the question some of my friends ask ...."what do you think my xxxx might be worth".Moreover it suggests that if the item you are interested in satisfies your individual audio needs, purchase of used gear that is 15-20 years old gives you a good chance of facing no further depreciation and perhaps even a small gain if you resell it within another 15-20 years.
Ask any questions, and let me know what you think.