The reason that this question is so important is that it can affect your choice of medium, digital vs traditional. Jim Lee said there were many new artists missing out on selling their original art work by working digitally. Granted, if you're a well known and sought after artist as Jim Lee, Todd McFarlane, Mark Silvestri, or Greg Capullo it would make sense to make more money from originals. New artists don't get the same recognition.
The trade off is losing your tools, shortcuts, and page rate for the aspiration to sell the original art for extra money. When you take into account your own notoriety and realistically calculate who would buy your original art you get a better idea of what your sacrificing for a very slim hope of additional cash.