The sparse column itself requires no space for storage if it contains a null value. However, additional storage space is required to store non-null values in the sparse column. That's why you don't get a 100% total space savings. In some cases the savings is pretty low.

http://blogs.technet.com/andrew/archive/2008/02/28/sql-server-2008-sparse-columns.aspx

Please correct me if I'm wrong.

The weekend is here... nice. 😎