splitting a time period into calendar months-Oracle SQL

Hi
I've got a table that holds a series of rows to do with billing periods & cost, like so:-

ACC_NUM      ST_DT      END_DT      DAYS      COST      AVG_DAILY_COST
264001440      07/16/2008      09/16/2008      62      103.65                 1.67
264001440      09/16/2008      11/13/2008      58      127.11                 2.19
264001440      11/13/2008      01/16/2009      64      205.78                 3.22
264001440      01/16/2009      03/20/2009      63      176.07                 2.79
264001440      03/20/2009      05/18/2009      59      125.09                 2.12
264001440      05/18/2009      07/20/2009      63      98.81                 1.57

When the number of days in the period is > 34, the cost has to be broken out by calendar month, with the following guidelines.

Example: Billing Period is 05/18/2009 to 07/20/2009 and the amount is 98.81 dollars.
Number of days in the billing period is 63 days. average daily cost is $1.57.

The amount assigned to May 2009 is 1.57 * 13 days = 20.41 USD.
The amount assigned to June 2009 is 1.57 * 30 days = 47.10 USD.
The amount assigned to July 2009 is 98.81-20.41-47.10 = 31.30 (subtraction avoids rounding errors).

There would also need to be a portion of the previous 2 month bill allocated to May 2009.

I'm envisioning this data transformed into a new table, with the account number and 12 columns, 1 for each calendar month.  
I'm not sure whether its best to label the 12 columns as the actual months, or to be non-specific, given that I'm dealing with a rolling 12 months.  ie will be a different data set each month during dev & testing  till the final cut-over in Jan 2010.

has anyone done something like this before?  I'd appreciate whatever help is on offer.

Many Thanks
Red