What is considered a "standard" percentage of devaluation per year foor high end speakers?

Here is what I paid for my DCM Time Windows 1A (my third pair!): List price in 1984 - $1200. I paid $219 + shipping from an EBay seller. So stuff can depreciate quite a lot! Other stuff (like a Marantz 7 and 8B) can go up a lot in value! 

I like Elizabeths formula....It had held true for a number of speakers I've bought and sold...  

There is no "standard" rate.

(1) In my experience the suggested 40% knockdown in year one can easily range to 50%, and do not necessarily plateau there with no further reduction ad infinitum..

(2) The variable depreciation in estimated fair market value, -- independent of what year out it is-- is also HIGHLY influenced by the state of the economy at the time you pull the trigger. Intuitively, expect a harsher grind-down in gloomier times.

(3) Thirdly, ALWAYS parse out a depreciated value for a dealer trade-in from any proposed private sale value. They don’t equate in any manner. The former is always going to be a much harsher mistress .... full stop.

another variable is how often the manufacturer changes models, or changes a speaker without a model change designation...some 7-10 year old speakers are still current, some have been upgraded/changed a number of times...