Here Costs are not incurred by IAM users, the AWS account awning the resources incurs the charge.
When the IAM users make requests to AWS, their credentials are verified just to ensure that they are authorized users and their permissions are checked to confirm that they have certain permissions to perform the operations. So there is no further relationship between the user making the request and the resource that is being created.
The closest connection is an audit log that is created by AWS CloudTrail that can show which user has made the request. but it is not related to the billing information.
If you want to track the cost, you need to associate tags with the resources.
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Also, you can go through the below tutorial to know more about IAM.