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


As the title of the thread indicates, what is a standard devaluation per year of high end speakers in generally  good condition. 

Often, on Audiogon, sellers ask for 50-to 65% of the retail price for speakers that are between 7-10 years old (or even discontinued) regardless, whether they paid retail or not. 

I realize that condition and the number of owners is a determining factor, and it should be. 

Thank you,  S.J.


sunnyjim
Post removed 
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.