greg.org: the making of: Diamonds Are Forever! TODAY ONLY!:
First things first: if someone DOES buy Damien Hirst's diamond-and-platinum skull, it won't be for $100 million. Any shlub billionaire walking in off the street would get 10% off, and any actual collector would get 20%. So if someone's been around the art block already, that's where he'll start talking.
Second, by floating a $20 million cost figure, Hirst is taking a page--and a number, even--from Christo's playbook that likely has more to do with setting a context for potential buyers than with the actual outlay.