Getting a the total differenace from month to month

I have been trying to find a way to get the total subtracting data from one workbook from another in excel 2014

Column B has the Model (Have many duplicates)
Column G has a MAC Address (Unique ID)
Column L has Page Count

The sheet is generated each month. So what is needed is to subtract column L from the previous month workbook, but need to match up the MAC if it exist still in the new other wise put NULL, because the printer was removed.

I was thinking I could do an array specifying workbooks in one sheet, with the formula on new primary sheet with: Printer Model, MAC Address, Month Count Total(August, September, October).

Example Names of the exported sheets:
Export08202014.xlsx
Export09202014.xlsx
Export10202014.xlsx

Not sure what function would work the best to get the data.

Thanks

Can you upload a sample file showing what you're trying to accomplish?

hoyasaxa215,

I have attached sample data. The file example_data.xlsx is the output I'm looking for. I have been trying to get VLOOKUP but getting lost on how to match to the MAC Address to make sure I'm not getting duplicates and only finding the unique rows.

The Export files have the data.

Thanks

-RM