Assuming S1 and S2 are separate files, not just separate
sheets in the same file, your index is column A, the costs
are column B, Row 1 is titles and the the sheet in s2 is
called Data, try this in S1, cell b2:

=vlookup(a2,'C:\My Documents\[s2.xls]!
Data'$A$1:$B$40000,2,false)

Amend the file path, sheet name, range reference and the
lookup reference (in this case ",2," for column B) as
required.

Pete