This short article will help you understand where to find the Billing Page.
Billing page gives you details about your Last invoice details and Current Balance.
You can go to Billing Page by going to Setting > Account Details > Billing Page.
Under Billing Page you will having an idea about your Current Billing cycle and Last bill paid. You can keep a track of your usage through it.
Current Balance tells you that if your account stays as it is right now, what the next bill amount would be and when it is due.
Last Invoice Details tells you about the various line items in your previous invoice and when the amount was deducted.