This is EXACTLY what the guy did, like a billion times! He made them multiply 0.002c/kB ("keep in mind that's in units of cents") with 35,000 kB...and they'd say it came out 71 dollars! Incredible! The mind boggles! No, it actually does a lot more than that, but I haven't found the words for it yet!Ivan's right, this would have been much clearer and much shorter, if he had just had them calculate what his bill should be.
Get the guy to say you are charged at .002c/kb, then ask him to calculate his bill.
3.5Gb x .002c = 71 cents.
Forget all the hypothetical stuff.