convert dates to calendar schedule showing how much to bill them each calendar month

[TheOne]

Ok, here is what I need. I have a sheet (as attached), that outlines a schedule of payments from a customer. The dates are setup something like this:
Start ---------Finish ---Price Per Month
12/1/2008 - 12/13/2009_ $100
12/14/2009 - 4/15/2010_ $126
4/16/2010 - 5/1/2010_ $0.00
5/2/2010 - 5/1/2014_ $500

All of these numbers must have the ability to be modified.

What I need is to calculate how much to bill them each calendar month. Such as:

Novemer 2008--December 2008 ... December 2009--January 2009
---$100------------$100---------$41.94+73.16------$126.00

Note that, as long as it is a full month, they are charged the full month (ie a month with 28 days is not less expensive than one with 31). However, if the month is split between two payment schedules, it's days determine how much to charge from either plan. Also, the months in the calendar should not be calendar, but manually entered.

I greatly, greatly appreciate your help!!!