Once you have created your cost monitor, you can choose your alerting preference by setting up a dollar threshold (e.g. only alert on anomalies with impact greater than $1,000) . You don’t need to define an anomaly (e.g. percent or dollar increase) as Anomaly Detection does this automatically for you and adjusts over time.
https://aws.amazon.com/aws-cost-management/aws-cost-anomaly-detection/