Find percentage of NULL storage in Oracle table
NULL value
  
  Absence of a value in a column of a row. Nulls indicate missing,
  unknown, or inapplicable data. A null should not be used to imply any
  other value, such as zero.


There are no alternate definitions of NULL in Oracle.  A column can't take less space than NULL unless the column doesn't exist at all.  Also, if a null value is in the last column of the row, the row will take less space than the same null column as an earlier column.  Here is the wording in the documentation:


  In general, try to place columns that frequently contain nulls last so
  that rows take less space. Note, though, that if the table you are
  creating includes a LONG column as well, then the benefits of placing
  frequently null columns last are lost.


Since in your example data, none of the rows with nulls have them last, we can ignore this space optimization.  We can also not compare the lack of a column to a column with nulls as that would be the true "space" of a null, which according to some block dumps I did is one byte per null per row.

In this case we are actually comparing the space of a null vs. the space of a value.  Because any value will by definition use more space than a null, my answer is that, a null value will decrease not increase the amount of space consumed, therefore creating a percentage compared to the size of the table is meaningless.

You could do a block dump of a row containing a value and then set the value to null and dump the block again to see how much additional space is available, but that number and the resulting ratio you could create with it and the size of the table would be a measure of space used by values rather than space used by NULLs.