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.