I don't think you'll find a good answer to that question, at least one based on factual information. The information is kept confidential for competitive reasons. As a passenger, there's really no way to know -- a full plane could be losing money and a near-empty plane could be raking in the money.

Another factor that comes into play is cargo, which I'm assuming can be a big deal (a former employer paid a small fortune to UA and LH for shipments of temperature-controlled pallets of pharmaceutical materials on FRA-ORD).