that should be simple, but I cannot figure that out
Amazon bills the real used storage amount for Aurora databases, not the allocated storage like for non-Aurora.
there is a metric VolumeBytesUsed - The amount of storage used by your Aurora DB instance. This metric applies to an entire Aurora DB cluster, not for instance. https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.Monitoring.html
I have a cluster of 1 Arora Writer and 2 Readers. DB size is about 10 Gb and VolumeBytesUsed shows 10Gb. According to documentation, each Aurora node has its own data copy - https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/Aurora.Overview.html So if I have 10Gb DB - Aurora Writer and 2 Readers should be using 10x3 = 30 Gb of storage, but VolumeBytesUsed shows 10Gb
Will I be billed for 10 Gb or 30 Gb in that case?
From the Aurora MySQL FAQ, which I found linked from the pricing page.
Based on what I know about AWS (quite a bit) and what I've read (not a lot) I think each database is charged once for storage. That means if you have a 10GB database service with two 10GB read replicas you'll be charged for 30GB of storage.
here is what I got from support: