Dropbox creates a hash identity for the file and compresses it in the cloud. So stuff like a song isn't uploaded or even stored twice. I assume it's still counted towards the usage.
I think the point wasn't about the cost to Dropbox so much as the opportunity cost (i.e. a lot more people would convert if the free package wasn't so good). Then again maybe if the free package wasn't so good, people wouldn't be such Dropbox fans.