It's also about the name of the artist - Damien Hirst is well-known.
http://en.wikipedia.org/wiki/Damien_Hirst
For example, Picasso could scribble some random doodles on a piece of paper - as long as you can prove it is an original Picasso, it would be worth $$$. If you and I did the same exact doodle, it would be worth not much at all.
Of course, an artist becomes well-known for a reason.