Tracking AWS costs by IAM User / Role / Group

I am aware of the ability to track AWS costs based on tags. However, this requires users to remember (and to know how) to tag the resources they use appropriately.

I'd like to know if there is a way to track expenses by which IAM User requested the resource(s) billed?

(Context: dozens of users, each with their own IAM key and various permissions; we have a separate bucket to store detailed billing, and a script that downloads the latest reports every 5 minutes and stores them whenever there is change, so we get good time-wise granularity. But we'd like to get better accounting on a per-user basis).