Well the definitive answer is that  it is worth what one can sell it for 
or what one will pay for it.
So if you have a slice of toasted bread with the face of Jesus (the guy 
who mows my lawn) on it and you can cozen  some fool on the Flea bay to 
give you a king's ransom for it then god bless you and  I hope you get 
another such piece of toast soon.

I have an old Walker Turner Driver line lathe for sale on the Flea bay 
and I have not had even a nibble so I'm guessing I am asking too much.