Cloud Sync business model for Windows 8 app ?

Hello,

I'm an indie developer that just got into Windows 8 Store app development (my first app Sticky List - download ) and I'm now planning a new application.

I plan on making the app free and charge $1 (one time fee, profit + server hosting) to enable Cloud Sync for that application (will also have an Android version) and wanted to ask the community (regular users) if this is a good idea or should I go with the ads model ?

If other users here have developed a Cloud sync application, is the $1 one-time-fee business model viable or I won't be able to cover my server bill (Amazon Web Services) once user start to heavily use the application?

Thank you for your time.