Thanks for everyones replies. This is incredibly frustrating and Air Canada / Aeroplan's lack of transparency is completely unethical.

The irony is ORD-YYZ-SCL or YYZ-SCL prices out to about the same. But the way they compute the fare is completely different. From ORD, the base fare is higher and the taxes / "fuel surcharge" is lower and vice versa from YYZ.

Maybe most people don't seem to care because the total price is about the same for revenue tickets. It seems only mileage tickets get screwed....!