Tying names to different values month to month.
 

I keep a list of top 20 costs for my department; I have two columns names of
the items and then the cost associated with each in the next column. That is
one month. The next month I have the same two columns but the items can vary
month to month based on their costs. I want to track the items costs so that
the values are next to each other and I can trend it better. How do I create
a function to bring over the values in my old spreadsheet where each top 20
is listed in a seperate group to a point where I have one running list of top
20 month to month to month? I tried IF functions but I am wondering if I need
to include in that logical test a LOOKUP function as well. Thank you for the
help in advance. I can send a spreadsheet for example if need be via private
email.