The easiest way for you to do this would be to Add Each day to its own section, making Monday's always A, Tuesdays always B, and so forth.
Then you go through (we will use 2 days ago till now as an example),
A2 Date of purchase 8/7/2009
A3 cost of purchase
A4 cost of purchase
A5 cost of purchase
A6 cost of purchase (final purchase of the day)
A7 Sum of A3 through A6 which would be written ignore the quotes "=SUM(A3:A6)
A8 Date of Purchase 8/8/2009
A17 Sum of A9 through A16 =SUM(A9:A16)
A18 Date 8/9/2009
A24 Sum of A19 through A23 =SUM(A19:A23)
A25 Monday Average =AVERAGE(A7,A17,A24)
This is the only way i can think to do it, you may even want to shift everything over 1 column, so you can put a labeling to the left of the sums so you know where they are, or just bold the text, but you will want to find some way to make them noticible. I know there is probably a far better way to do this, but I am not experienced enough with excel to know how.