I know how much I spend every week, but not how much I spend every day. I'd like to be able to find the total amount spent each month. Is there a way to calculate this automatically?

The problem is that the month does not divide nicely into 7 day weeks, so for instance in the following data to find the total for November I could manually write a formula to sum 153+207+353 + (99/7*4) + (203/7*4) but I'd need to write a new formula for each month depending what day of the week the 1st fell on. Is there a special function or any way of doing this automatically with a single formula for every month?

29/10/2012 £99.00
05/11/2012 £153.00
12/11/2012 £207.00
19/11/2012 £353.00
26/11/2012 £203.00