I was surprised how incredibly difficult that is to set up. Eventually I dead-ended following the instructions when CloudWatch told me there were 0 metrics to choose from for monitoring...
There are a couple of things that are easy to overlook when using billing metrics.
1. All billing metrics are stored in us-east-1 even for usages in other regions.
2. If you are using consolidated billing, billing metrics will be published under the linked account, and will only be visible to that account.
Hope that helps.
...what? Is this true? If so, can someone explain the logic behind this?