It's worth as much as someone is willing to pay for it...
McIntosh amps still sell routinely over $1k 25 years after they were made, and often times in horrible cosmetic condition, because there's a demand there. Not that they're that much better, but because the following is there to drive prices up.
Speakers tend to take the greatest hits in value, because people often times don't set them up properly and shorten their lifespans, or they're subjected to all sorts of abuse from weather, screwdrivers, shifting cargo, competition, etc...