How the Detailed monitoring is actually charged.

Hi Ophelia,
For detailed monitoring you are charged per metric that is sent to CloudWatch. You are not charged for data storage. For more information https://aws.amazon.com/cloudwatch/pricing/. You can see the various tiers associated with the costs, navigate by region (pricing can vary by region) and see some pricing examples as well. All custom metrics charges are prorated by the hour and metered only when you send metrics to CloudWatch.
So for example, for EC2 Detailed Monitoring if your application runs on 10 Amazon EC2 instances 24x7 for a 30-day month, and you enable EC2 Detailed Monitoring on all instances, your charges would be as follows:
Total number of metrics = 7 metrics per instance * 10 instances = 70 metrics
Monthly CloudWatch Metrics Charges @$0.30 per custom metric = 70 * $0.30 = $21
Monthly CloudWatch charges = $21 per month
With regards to what you said you saw in Billing under CloudWatch, the number 12.544, it might be that some metrics got deleted as it was no longer in use as a result you only pay for the time it was active.
I hope this helps.