Consolidated billing is now a feature of AWS Organizations. The information provided in the Billing and Cost Management documentation might be out of date. For current information about consolidated billing, see Consolidated Billing and AWS Organizations in the AWS Organizations User Guide.
For billing purposes, AWS treats all the accounts on the consolidated bill as if they were one account. Some services, such as Amazon EC2 and Amazon S3, have volume pricing tiers across certain usage dimensions that give you lower prices when you use the service more. With consolidated billing, AWS combines the usage from all accounts to determine which volume pricing tiers to apply, giving you a lower overall price whenever possible. AWS then allocates each linked account a portion of the overall volume discount based on the account's usage.
The Bills page for each linked account displays an average tiered rate that is calculated across all the accounts on the consolidated bill. For example, let's say that Bob's consolidated bill includes both Bob's own account and Susan's account. Bob's account is the paying account, so he pays the charges for both himself and Susan.
As shown in the following figure, Bob uses 8 TB of data transfer out during the month, and Susan uses 4 TB (for a total of 12 TB used).
For the purposes of this example, AWS charges $0.17 per GB for the first 10 TB of data transfer out used, and $0.13 per GB for the next 40 TB used. This translates into $174.08 per TB for the first 10 TB, and $133.12 per TB for the next 40 TB (remember that 1 TB = 10244 bytes).
This means that for the 12 TB total that Bob and Susan used, the total amount that Bob's paying account is charged is the following: ($174.08 * 10 TB) + ($133.12 * 2 TB) = $1740.80 + $266.24 = $2007.04.
The cost-per-unit of data transfer out for the month is therefore $2007.04 / 12 TB = $167.25 per TB. That is the average tiered rate shown on the Bills page for each linked account on the consolidated bill, and in the downloadable cost report.
Without the benefit of tiering across the entire consolidated bill, AWS would have charged Bob and Susan each $174.08 per TB for their usage, for a total of $2088.96.
Note that Amazon SimpleDB in particular has a free tier, so we apply that free tier to the total usage across all the accounts; we don't apply the free tier to each account's usage. For more information about Amazon SimpleDB billing tiers, go to the Amazon SimpleDB product page.