Speaker depreciation: Is there a rule of thumb?

I'm looking at used speakers and wondering if there is a 'rule of thumb' for determining a fair price. I imagine (operative term...imagine) what kind of reviews the speaker received after manufactured, materials used, age and other factors have an impact. Your thoughts?
There are an awful lot of variables so any rule of thumb is just a guideline. I have the same rule of thumb for pretty much all high end equipment, not just speakers. So here it is.

I expect to see depreciation in the 25%-33% range for equipment that is up to two years old. By two years old, it should be depreciated by a third. I then find that high end equipment will hold its value pretty well at roughly 50% from year 2 to year 5 when the warranty typically runs out. After that I expect another significant drop to about one third of its original price. Adjustments are made to these figures for physical condition.

That's a pretty gross generalization, but it is what I notice. For a particular brand or item, if you're talking about spending a lot of money, it would make sense to follow listings on the used market before you buy so you know what prices to expect. It might also be a good investment to check out the AudiogoN blue book.

I'll be interested to see what other posters think is a good rule of thumb.
The rule of thumb is never to buy new if you can buy the equivalent second hand.Hi Fi gear can depreciate rapidly.If I buy second hand I look to buy it for a third of its new retail price and if the product is more than 3 years old this is very often possible.It needs to remembered that when people come to sell second hand they like to quote the retail price but rarely does anyone actually pay this price.

I have noticed that the depreciation in addition to age is size and brand dependent. Big unknown speakers you are lucky to get 50% more likely 30% of oringinal msrp. Small easy to ship speakers from a well liked ,popular brand, you see a lot of people asking for as much as only 30% off of msrp, just the opposite.
popularity rules hear. buy used and avoid the pain.
Markphd makes a good suggestion in my experience: Follow the listings for a while. I have found the AudiogoN blue book to be generally high in average price. I am currently watching the Sony SCD-1 market, and missed a good one because I had not updated my email address on AudiogoN so the seller missed my what-would-have-been-winning bid.