I've been charged over $800 in excess baggage on some trips. That was for two boxes (total weight, about 55kg), and a regular suitcase, and that was on Business Class.

Excess baggage can really hit you, if the people behind the counter want it to. That luggage I described above usually gets hit for $200-$300 excess baggage, but roughly 25% of the time, they don't charge me anything.