We have just released Billing Alerts for AWS, our new application for iOS and Android. It was developed to track Amazon AWS cloud usage costs in a simple way, by receiving notifications on the phone whenever the current billing amount increases by a chosen interval. For example, when using the default interval setting of $10, you will be notified whenever your current Amazon AWS bill goes over $10, $20, $30, etc.
Please see the Billing Alerts for AWS Product Page for more information and details on how you can configure your AWS accounts automatically or manually.
You can find Billing Alerts for AWS (iOS) in the iTunes App Store, and Billing Alerts for AWS (Android) in the Google Play Store.