Null is neither zero nor empty string. Null is not a value at all. Most importantly for our discussion, one null value does not equal any other null value. In the RDBMS, Null simply means a value that is not known.
NULL value can occupy in the database based on the coulumn data type and width.
Fixed length data type NULL value takes the space as width of filed. (Char (5) – NULL value take 5 bytes)
Variable length data type NULL value takes 2 bytes. (varChar (5) – NULL value take 2 bytes)
Integer data type null value takes 4 bytes space
You can use the sparse columns to save the space of NULL values. http://technet.microsoft.com/en-us/library/cc280604.aspx