You should just check on eBay to see if there are any out there of the same make/model and go with what they have sold for as the real world value. Generally speaking, speakers can wear out and even good car speakers drop in value over time significantly. I wouldn't expect much for them.
I had a very nice Polk 10" custom built car subwoofer that I sold for about 1/10th of what I paid for it in the box. Maybe less. The value just doesn't hold up except, perhaps, on the absolute highest end items, and eBay will help you know if that's what you have.