Which Windows and/or Windows Phone app business model suits an app with a Microsoft Azure backend?

It would seem that an app purchased from the Windows Store for, let’s say, $1.49 (which is one-time revenue) would quickly be consumed by the continuous (variable) cost of the cloud services it interacts with. What is viable? Ads? In-app credits that get consumed and replenished based on backend usage?

Can a cloud-enabled app support itself financially?

Reference links:

• http://msdn.microsoft.com/en-us/library/windows/apps/hh465433.aspx
• http://social.msdn.microsoft.com/Forums/windowsapps/en-US/8b78d62d-e8d5-4928-9ebb-67d5f4a907c3/subscription-modelsupporting-azure-costs?forum=windowsstore#8b78d62d-e8d5-4928-9ebb-67d5f4a907c3
• http://social.msdn.microsoft.com/Forums/windowsapps/en-US/19b9e92f-b1fd-4eb1-8133-4bf98fc9a0e9/payment-model-for-a-cloud-app-with-azure-backend?forum=windowsstore#19b9e92f-b1fd-4eb1-8133-4bf98fc9a0e9