tough question
it seems as though this is a predominately market driven affect, so to ignore how the market determines the price would be fool-hearty.
I think it boils down to the 1) popularity of the speaker (reviews go a long way here, irregardless of right or wrong), 2) # of models currently on the market or 3) whether it is the latest hot item or hot item of yesteryears (which often see pennies on the dollar).
I hate to say it but audiophiles are sheep too. There are a few that lead and many that follow. If someone has the latest and greatest and says that their X is not as good, soon we'll see many X's on the market and a flood of orders for the latest and greatest. As much as you will see a million justifications for why this is not so, the history bears this out. Of course supply has a large part of it too. (if you want to research more on the topic, do some digging on 'herding behavior & probability').
I am not trying to embarrass anyone or be judgemental but I am trying to describe how the dynamics of the audio second hand market work. I could numerous items that were highly demanded and bid up only a couple months ago that are taking large baths in the secondary market today.
When a company comes out with a 'new and improved' model, the later models will still sharp and sudden depreciation. What drives some markets on ebay for NOS goods, while other NOS goods of similar nature go for pennies? Same thing going on here. People don't want to be bothered with sending things in to be updated, they want it as of the latest for a lot less than that currently costs.