Do you know the storage needs for nvarchar(X)?
So for instance, when the value inside a column is a lot more compact than X, just how much is really saved within the database page?
An nvarchar column is how big the amount of unicode figures saved inside + an overhead. The precise dimensions are most likely dependant from the SQL server being used.
For Microsoft SQL server "the storage size, in bytes, is 2 occasions the amount of figures joined + 2 bytes". See: http://msdn.microsoft.com/en-us/library/ms186939.aspx
Based on Microsoft:
"Storage size, in bytes, is 2 occasions the amount of figures joined."
In fact, there's an additional 2-bytes used beyond that as overhead to exhibit length.