How to split NW costs among accounts using Centralized Network architecture

Any advices on how to split NW costs among multiple AWS accounts when using an Hub and spoke design with AWS Transit Gateway?

https://docs.aws.amazon.com/whitepapers/latest/building-scalable-secure-multi-vpc-network-infrastructure/transit-gateway.html

Since TGW has data processing costs, how can we identify NW traffic generated by accounts and split billing accordingly?

The same applies to Centralized egress to internet, where we have NAT gateway(s) serving traffic from multiple accounts. Any recommendation on how to generate this accounts traffic analysis from a cost perspective?

Thanks a lot