91B...What is it worth ?

I have a 91B. Probably made in '92. What is this worth ? 
   Also have an Etherwave signed by Bob.

   I would love to own either one, especially the 91b.  Couldn’t offer cash but would trade for other instruments.  I have collected many different instruments over the years but have come to Theremin late in life and love it.  Many guitars and other instruments of all types if interested in trade.  If not, good luck and hope the Theremins find a place to be played.
Have a great day