The difference between a $50,000/year employee and $500,000/year employee

Maybe I don't have enough experience/knowledge about this but what makes a person worth $500,000/year versus someone making $50,000/year. Before you say something basic like "The $500,000/year employee has more experience/responsibilities please keep in mind that the opportunity cost for 1 $500,000/year employee is 10 $50,000/year employees so the banks employing these $500,000/year employees is inferring that the 1 expensive employee has the same productivity as the 10 cheaper employees combined. So the question brings us back to: what makes a single person worth a salary of $500,000/year and why can't these employers find a perfectly qualified person to do the same job for a small fraction (let's say $150,000/year).