I don't think it would be possible giving it a numeric value, given what it entails & what danger it is to the world. As Minamoto Teh First has pointed out, one would have to be a full fledged moron to price it, or even put it on auction, so the world can be damned by one person who will carry out his own sense of justice with it. The only real value I could give it is one I would give to life, and only a little less because a Death Note doesn't have a soul, while it can take as many souls to hell/heaven as much as he who possesses it can write down the names of whom he wishes to kill. i.e before an L replica emerges from humanity's population and hunts the book owner down.