Originally Posted by HerbFlyer

Originally in our saga we bought W fare tickets for us (so we could use GPUs). I think at the time of the *real* ticket for our son being issued (approximately two weeks later) the lowest available fare published -- and available on our flights -- was a K fare, and it seemed to be based upon that. In other words, all three of us paid the same customs fee taxes but his 10% fare was about 7% of the adult tickets...

The agents won't give you a straight answer on this- they really don't know. If they do, don't trust it. You could get the rate desk on the phone to answer it specifically if you want... but in the end for an economy ticket it'd only be a few $ difference. For upgraded fares is where the numbers mattered a lot more to us.