I really, really advise against this approach of over-riding cells. Why not just list out your daily expenses in a column (adding new expenses to the bottom of the list) and sum by month (using a pivot table or sumif or whatever your method of choice is)?
As an example, say you put in $225. Then, in the next entry, you put in $100. But, you decide that the item before should be reversed out. Did you put in $255? Or $225? "Shoot, I can't remember!"
Or, "did I put in both of these $225 receipts before I got my phone call and interrupted my train of thought?" Stuff like that will be so much easier if all of the items are (transparently) listed instead of just being able to see the last adjustment and the total.