Let's face it... there are darned few careers where you can name your own salary or fee. And pro speaking is one of those exclusive careers.
There are no salary caps on what pro speakers can be paid, and there are no outside agencies regulating your fees. You can charge whatever you want, and you can raise your fees without anyone's approval - assuming, of course, that the people who hire you agree that you're worth what you're charging.
That means that if you think you're worth $1 million to give a speech - and you can convince other people that you're worth that much money - then you're in a position to make more money with a single speech than many people make in a lifetime.
(Now, to be honest, only a few speakers ever become millionaires. And you will have to work your asterisk off to make those megabucks. But the point is, it's your decision how much you can charge... not the government, not a union, and not a regulatory agency.)
In short, nobody can limit how much money you can make as a pro speaker... except you!