Just for a remarkably specific situation, but-
China-Japan fares really get out of hand sometimes. I've once paid over $600 for Shanghai-Osaka in Y. For comparison, Nashville to Newark isn't much shorter and often be found for half as much. But when US Airways is having a promotion on buying miles, the purchase price usually works out to be about that much for a ticket in C, essentially a free upgrade if I'm planning on going on an off-day.