Speaking of vocoders, being a dork for all things electronic music, I have in my possession a vintage Roland VP330 vocoder manufactured in 1981. I don't think a better one has ever been made.
Very nice! Now if I could just figure out how to use the thing!
Here's something I picked up at a garage sale for $5
I have no idea what (if anything) they're worth but it is fun to play with.
If it functions it's definitely worth something. Is yours the tube or the solid state model? I read up on those a while ago, and IIRC there were 4 models - 2 tube models, and later, 2 solid state models. Of course, it's the tube models that are the most desirable for audiophiles, and likely carry the most value. I have no idea whether you're talking hundreds or thousands, but any way you slice it, a functioning Echoplex is going to be worth many times over the $5 you paid.
Garage sales can be a great place to score big! People just don't know what they have sometimes.
ETA: I just did a cursory search and found
THIS history and overview of the Echoplex devices. It may give you some clues as to the value of yours.