Value of Service Rental over time

[Doneill]

Hi Gents & Ladies,

I have a bit of a head scratcher and I am trying to run before I walk 

I am looking to create a measure on a date table in power pivot that will calculate the rental values of a service in a given month.

My Fact Table Example:

1. If End Date is Blank it is still billing
2. If End Date has data there would be no charge

So I need the dax formula that would allow me to show the billed value in a given month, and then show this in a timeline, YTD etc.

Any Gurus out there able to point me in the right direction?

Thanks in advance.