If the field is fixed width storing NULL takes the same space as any other value - the width of the field.

If the field is variable width the NULL value takes up no space.

In addition to the space required to store a null value there is also an overhead for having a nullable column. For each row one bit is used per nullable column to mark whether the value for that column is null or not. This is true whether the column is fixed or variable length.

The reason for the discrepancies that you have observed in information from other sources:

• The start of the first article is a bit misleading. The article is not talking about the cost of storing a NULL value, but the cost of having the ability to store a NULL (i.e the cost of making a column nullable). It's true that it costs something in storage space to make a column nullable, but once you have done that it takes less space to store a NULL than it takes to store a value (for variable width columns).
• The second link seems to be a question about Microsoft Access. I don't know the details of how Access stores NULLs but I wouldn't be surprised if it is different to SQL Server.

The following link claims that if the column is variable length, i.e. varchar then NULL takes 0 bytes (plus 1 byte is used to flag whether value is NULL or not):

• How does SQL Server really store NULL-s

The above link, as well as the below link, claim that for fixed length columns, i.e. char(10) or int , a value of NULL occupies the length of the column (plus 1 byte to flag whether it's NULL or not):

• Data Type Performance Tuning Tips for Microsoft SQL Server

Examples:

1. If you set a char(10) to NULL , it occupies 10 bytes (zeroed out)
2. An int takes 4 bytes (also zeroed out).
3. A varchar(1 million) set to NULL takes 0 bytes (+ 2 bytes)

Note: on a slight tangent, the storage size of varchar is the length of data entered + 2 bytes.

From this link :
Each row has a null bitmap for columns that allow nulls. If the row in that column is null then a bit in the bitmap is 1 else it's 0.

For variable size datatypes the acctual size is 0 bytes.

For fixed size datatype the acctual size is the default datatype size in bytes set to default value (0 for numbers, '' for chars).

Storing a NULL value does not take any space.
"The fact is, a NULL value occupies space – 2 bytes."

This is a misconception -- that's 2 bytes per row, and I'm pretty sure that all rows use those 2 bytes regardless of whether there's any nullable columns.

A NULL value in databases is a system value that takes up one byte of storage

This is talking about databases in general, not specifically SQL Server. SQL Server does not use 1 byte to store NULL values.

Even though this questions is specifically tagged as SQL Server 2005, being that it is now 2021, it should be pointed out that it is a "trick question" for any version of SQL Server after 2005.

This is because if either ROW or PAGE compression are used, or if the column is defined as SPARSE, then it will "no space" in the actual row to store a 'NULL value'. These were added in SQL Server 2008.

The implementation notes for ROW COMPRESSION (which is a prerequisite for PAGE COMPRESSION) states:
NULL and 0 values across all data types are optimized and take no bytes1.

While there is still minimal metadata (4 bits per column + (record overhead / columns)) stored per non-sparse column in each physical record2, it's strictly not the value and is required in all cases3.

SPARSE columns with a NULL value take up no space and no relevant per-row metadata (as the number of SPARSE columns increase), albeit with a trade-off for non-NULL values.

As such, it is hard to "count" space without anlyzing the actual DB usage stats. The average bytes per row will vary based on precise column types, table/index rebuild settings, actual data and duplicity, fill capacity, effective page utilization, fragmentation, LOB usage, etc. and is often a more useful metric.

1 SQLite uses a similar approach to have effectively-free NULL values.

2 A brief of the technical layout used in ROW (and thus PAGE) compression can found in "SQL Server 2012 Internals: Special Storage" .
Following the 1 or 2 bytes for the number of columns is the CD array, which uses 4 bits [of metadata] for each column in the table to represent information about the length of the column .. 0 (0×0) indicates that the corresponding column is NULL.

3 Fun fact: with ROW compression, bit column values exist entirely in the corresponding 4-bit metadata.

At the time of this posting, the most upvoted answer Answer by Mark Byers is mostly correct, but it also states that...
"If the field is variable width the NULL value takes up no space."

That's true only some of the time and the cases where it's true are actually quite rare because of the column order people choose for their tables.

So, the correct answer for SQL Server is, "It Depends".

To summarize (and simplify using only VARCHAR as an example)... NULL VARCHAR columns take ZERO bytes ONLY if there are NO VARCHAR columns to the RIGHT that are populated (not NULL, not "Empty String"). Any and all NULL VARCHAR columns to the LEFT of a populated column will take 2 bytes each for the "Column Offset", which marks the "end byte" for each variable width value in the row.

That means that if you have 10 VARCHARS in a table that are all NULL, they all collectively take ZERO bytes. Add just 1 character to the right-most column and your row expands by a whopping 23 bytes.

2 for the Variable Width Column Count 
20 for the 10 two-byte "Column  Offsets"
 1 for the actual byte added to the right-most VARCHAR
--
23 total bytes added to the row as a result of adding just 1 byte to the right-most VARCHAR.

Here's the link to an answer here on StackOverflow that shows what happens at the row level using an easy to read, formatted output from DBCC PAGE.

https://stackoverflow.com/a/73439645/313265

NVARCHAR works in an identical manner for NULLs and "Empty Strings". I have NOT tested with other variable width datatypes but my "unqualified opinion" would be that they would work the same way.