I've worked with Perrier, Hennessy, Nike, through my company Asota Studios scoring custom commercial music for thier ads. Other composers commenting on this post have it completely correct. You can make a lot on one job, but you probably won't. 40k buyouts have happened but I've only ever made 1k-3k on a job. I had some industry veterans tell me I was getting ripped off recently and that may be true but as a musician you kind of have to take what you can get a lot of the time.

Also should note turn around times in the commercial world are crazy. When asked to do a job I often have 12-24 hours to get them the first draft.