To do what you want you always need to know the filename, but here is what I have

=LOOKUP(A2,'[sampleoriginal.xls]original file'!$A:$A,'[sampleoriginal.xls]original file'!$C:$C)
A better way of doing this is to create a template file that compares 2 data sets on one sheet, and pulls the new set with prices to a seperate sheet, then just copy that out to its own workbook and save it out every time you work it.