Thanks for the illustration, Moondog. I'm actually a number cruncher by profession ; below are the actual numbers if anyone is interested:

The receipt states the AUD amount is $685, which Virgin Blue converted to $628.80 USD after a 3.75% margin. Therefore, the amount I should have been charged without the margin would be $606.07 USD (= 628.80 / 1.0375), and the margin they should credit back to me is $22.73 USD (= 628.80 - 606.07).

Or equivalently, $25.69 AUD (= 685 * 3.75%).