Using Excel 2003.

I have two columns for each month in a spreadsheet. Column A is the item
code and column B is the cost/item for last month. i then have in column C,
item codes and in column D cost/items for this month. Columns A and C do not
match entirely, because the same items may not have been touched each month.
Each may contain unique items, but some are common. I need to do the
follwing:

In column E, I need each row to look at what is in column C, see if it is
also in column A, and then show the decrease/increase of cost/item
(difference between columns B and D).

MARCH APRIL CHANGE IN COST
A B C D E
A134 $1 A043 $2 (SHOW NOTHING OR $0)
A558 $3 A134 $2 (SHOW +$1)
A6563 $2 A6563 $2 (SHOW $0)
A779 $2 A694 $4 (SHOW NOTHING OR $0)
A965 $8 A779 $1 (SHOW -$1)

I hope someone can help -- is making me crazy!