You'll probably get an answer here, but I think you'll get faster results if you go on eBay and do a search for the brand and model number of your speakers.
If there are any listed they will come up. Of course what people are ASKING is one thing; what they GET is another.
THAT'S WHY YOU WANT TO LOOK AT....COMPLETED ITEMS.
The completeds will show you what they sold for. Just look to the left side of the screen after you have done the initial search. About 4 to 6 inches down you should see a link for COMPLETED ITEMS (or completed listings, don't remember for sure which it is.)
OF COURSE I REALIZE this is NOT THE ONLY ANSWER or solution. This is what I do and IT WORKS FOR ME. This is my opinion. Other people will have their own opinions which have value as well.
Edit: addition....you can also do a search on Craigslist for your item. If they don't come up, switch to a larger city in your state or even a different state with a large city. Of course this will just give you some idea of value, but you have a starting point.