How about this for starters. Say you have 3 worksheets named 'Compare' '2011' and '2010'.
Assuming you have all your data for 2010 and 2011 data only through October, try this formula in the Compare sheet, cell D6 and copied down for the 12 months:
Code:
=IF('2011'!D6>0,'2011'!D6-'2010'!D6,"")
If something more than this is needed and you need additional help, provide specific details of what you are comparing and where the information is stored.
Your example has a single sheet. Do you want to compare data between workbooks?