Storage cost of a null column

What is the storage cost of a null entry in a column? i.e. does a null
entry of type integer, float8 or text consume the same amount of
storage as one that is filled? I ask because I have satellite data
which is transmitted via a dodgy RF link that drops data packets. This
means I have a number of columns in a table that are null. Moreover,
the operations people decided to use a compression scheme whereby
non-changing bit/integer values are not output at regular intervals
which also adds a considerable number of null entries into the columns.
Because of this, we made a decision that we would have hundreds of 2
column tables (timestamp, value) and use unions, intersections, and
joins to get what was needed. Unfortunately, this has made application
programming a real nightmare as we are often forced to reconstruct a
snapshot frame for the range of times either in C or have the app
create temporary tables in SQL and insert the relevant data prior to
selecting it. As it stands, we've ordered a new disk array and
provided that the storage costs are not that high, I will probably be
reorganising all this next week. If anyone has any other suggestions,
I'd be very keen to hear them.

Cheers,
Randall