Napkin art

A couple years ago, I etched a couple shapes on a napkin, and bet with a friend that I would one become famous enough it would sell millions.

napkin-sketch

Unfortunately, I have not yet sold this napkin millions. However, I still have it, and I plan to hold on to it more dearly than shares on the stock market.

What valuation strategy would you suggest for this vintage black and white piece of art?