There are several sources from MS or MS related that specifically say storing a null takes NO space
http://blogs.technet.com/andrew/archive/2008/02/28/sql-server-2008-sparse-columns.aspx
http://msdn.microsoft.com/en-us/library/cc280604.aspx
Specifically from the link above, a little aways below the 'less space' item quoted in the thread above, you find this (emphasis added)
The SQL Server Database Engine uses the SPARSE keyword in a column definition to optimize the storage of values in that column. Therefore, when the column value is NULL for any row in the table, the values require no storage.
So if you read the details, the 'reduction' for storing null values is apparently 100% 'eliminate' would perhaps have been a more accurate word for them to use than 'reduce', since most of us think of reduce as being less than a 100% modification.. Still since it's proper to speak of 'reducing a price to zero' I believe it's technically grammatically correct to say 'reduce' in that instance, and NOT a conflict to then later in the same document spell out the extent of the reduction being 100%.