Put the description into your search engine--sometimes the answer comes up as one for sale on ebay or somewhere else; you can tell what people are at least asking for it. Also check to see if ghost of the robot has anything signed and how if it added value to whatever it was. Even autographs, how much are they? How much is just the guitar worth, unsigned? Once you find that stuff out you may have a better idea of what you might charge. Or stick it on ebay and let the bidders duke it out.