So this is currently not possible for two reasons, Tableau does not have the ability to give data sources aliases, and calculations cannot use alias values.
Are your data files text files? If so, I recommend creating a wildcard union. A wildcard union means that new text files added to the same folder will be automatically added in Tableau the next time the connection is refreshed (there is some flexibility around the logic you can use for which files are included).
AND if all of your data is unioned then there will be only 1 [Total Cost] field. Depending on exactly how your view is set up the exact calculation will differ, but you can probably use something like:
SUM([Total Cost]) - LOOKUP( SUM( [Total Cost] ), -1 )
The above calculation will subtract the SUM([Total Cost]) from the previous row/column from the SUM([Total Cost]) in the current row/column.
Hope this helps