My take is that it’s already your systems feature, rather than admins responsibility. If you treat departments like customers, you’d find a good way to spread the costs. If something is just a „common infrastructure”, you will always find something that makes costs that doesn’t have an easy way to track who triggered that - because you don’t pass enough information with it.

Not sure what is hard in it - you need consistent tagging, and that by itself gives you a lot of mileage in cost explorer.