Hi Dave,
Feature storage Credits are charged hourly based on a monthly rate. I'm going to provide some information about how to calculate the hourly storage costs, but it's not necessary for you to do the credit math. If something seems strange, take a look at the storage sizes of each feature layer through this reporting tool:
Understanding Feature Storage Reports (December 2016) | ArcGIS Blog
This Learn ArcGIS Course also includes some Credit Math if you want to look into this further:
Monitor member activity—Set Up an ArcGIS Organization | ArcGIS
Either way, if something seems strange, feel free to give your account rep or support services a call and they can review the report with you.
Credit Math:
Example if you have 10 MB of feature storage for an entire month, it will cost 2.4 credits for the entire month.
If you publish and delete feature storage throughout the month, the credit charges reflect these changes due to the size of feature storage scheduled hourly. This isn't the exact math as I have rounded the numbers and estimated the month, but should provide an idea about how the hourly storage is calculated.
Example if you had a 10 MB service and had it hosted in ArcGIS Online for 6 hours, it would be
2.4 credits per month ÷ 744 hours (month of March) = 0.00322 (credits per hour)
0.00322 credits * 6 = total of 0.019 credits for storage for the six hours.
This is calculated based on the total storage in your organization which can be viewed in the view status dashboards.
Let us know if you have any questions or if there is something that you want more insight into that isn't provided through the reporting tools.
-Kelly