How much Azure storage am I using for my DPM backup

Looking in your Data Protection Manager console you see that you are using X amount:

 

You then login to the Azure Dashboard and see that the same amount is being reported:

Switching to your Azure Account details you seen a different amount:

 

 

The reason is because of the amount of de-duplication that the Azure Cloud performs as detailed here:

Will the number of GB I am charged for be exactly the same as the amount of data I am backing up from my on-premises server?

No. There are several factors that will impact the amount of storage you are consuming in the Backup service including, but not limited to, compression ratios, the rate at which the data changes, and the number of backup copies you elect to retain in the service.