That seems impossible to determine. AWS retail charges for things that have an impossible relation to actual costs (think traffic) to begin with.
AWS knows how much it costs to deliver their own service. How do you think they determine their own profits and prices? They use those same calculations to bill internal customers.
Obviously AWS knows how much it costs to run all of AWS. But that doesn't translate to accurately knowing the marginal costs of a gigabyte of outbound traffic. They probably do know, but it isn't necessary at all to determine their profit, or to set their prices. "Just" set prices to what the market is willing to pay, and determine profit as total income minus total expenses over the whole operation.
This is totally untrue but keeps getting repeated. Yes, within Amazon, the non-AWS business units pay AWS to use AWS. But they pay cost plus a small percent, not retail rates.
The cost to non-AWS businesses is the same as if they had to do it all on their own (actually a little less since they get to leverage AWS's economies of scale).
The profitability of the other business units is actually improved this way. They would be paying more if they were independent and doing it on their own. This is why Amazon is so allergic to spinning out AWS as its own business.