How much space is one row taking up
How much data do you have?

Personally, I'd do one of


try it on a test system and check "before" and "after"
just remove unused data because by definition, it isn't needed


The questions is complicated when you have


variable length data: how much of the field is used on average?
page density (rows per page) may not increase therefore you save no space
indexes duplicate a column value, so you have to count some ones twice


So, I'd do it empirically rather than by calculation...