microsoft excel - IF Statement with life of contract dates, total amount and monthly write off

I am trying to create an IF Statement that based off the start date, life of the contract and Prepaid amount it will populate accordingly in the correct month.

Example: start date is 7/1/2017, amount is $911.97 and life of contract is 3 months. How do I get each month to reflect the correct monthly cost for that month if any, based on the life of the contract?

If just for 3 months Oct and on should have zeroes.

Example:

THANKS FOR YOUR HELP!