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.

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.