In any normal social system you care to examine, the gap between the poorest employee and the richest is about a factor of 20.
Do you make this assertion with evidence or without evidence?
This includes the Roman Empire,
Wikipedia says "At the time of Augustus, as many as 35 percent of the people in Italy were slaves..."
Not much point looking up a 1040 for Augustus when a third of the populace received no salary.
This is hard to evaluate. The first hit I found searching for "salaries in the catholic church" gives an average minister pay of about $35,000. Presumably the minimum would be significantly lower, especially if you count remote parish priests working for next to nothing.
The first hit for "salary of pope" claims an income of $200,000,000 per year for Benedict XVI. The second hit says the amount is $0. Hard to find a 1:20 ratio here either.
This is also hard to judge, but might be close to 1:20 if you exclude athletics and use the median instead of minimum on the low side. Here's a scandal about a professor making $700,096 a year. Another article shows Yale's president making over a million (a fairer comparison than the far better-paid football coach, given your mention of the U.S. president). A Harvard professor makes $1.19 million.
How do you figure out the minimum? An adjunct professor in Vermont earned $24,000 one year. The median salary of an English language instructor was $34,712 in 2005.
Here the ratio is closer to 1:10, but seems a stretch to describe it as a "normal social system" when it is more like a single organization.