Automating monthly budget averages based on start and end date

Hello, I am trying to create a document that takes our client PO amounts, tactics/spend line items, and dates from one sheet and averages them across the months that it is eligible, so we can know the total amount of money we can expect to go out from our clients each month.

For example:
Client1 Tactic1 StartDate EndDate TotalAmount
Client1 Tactic2 StartDate EndDate TotalAmount
Client1 Tactic3 StartDate EndDate TotalAmount
Client2 Tactic1 StartDate EndDate TotalAmount
Client2 Tactic2 StartDate EndDate TotalAmount

I have mostly figured this out in a really complicated way with many nested if statements, but there HAS to be a simpler way. There will be lots of hands in this document, so I would like to make it as simple and easy as possible.

Please see attached for a more in depth example.

Thanks!

I guess you found a way to add it all up if you have easy dates. I got it to work with clean dates such as 01.01.2014 to 30.04.2014. But with 15.01.2014 it gets very complex with the formula I tried

I can separate the dates with a left/right formula if needed (that is what I was originally doing the in hidden columns). It definitely gets complex with the dates that start in the middle of the month. If we can't do those and need to just round them to the beginning of the month that is fine. But, ideally it gives you part of that month as well.