There is another bin that is "paid for brand-building advertisement." - i.e. celebrities.
The proposition is that ex-government officials are just getting paid market rate for her knowledge and insight.
If that's the case, it should be trivially easy to find non government officials people getting paid $300k a speech for their knowledge.
You've identified listings - which is better than I thought there would be ! - but not sales.
Yellen earned $7.2m in two years. If all three of those combined earned half as much in speaking fees over that same time, I'd be definitely convinced that it's ho-hum
If you're a motivated research reading this interested in answering this empirically, I'd suggest trying to find organizations that both pay this level of fees and have to disclose it to the public when they do (I'm hoping this might be the case for public companies under SEC regs, but that's just a hope)
I am struggling to think that there are any examples that you won't dismiss, no matter how applicable.
Dan Schulman, Dara Khosrowshahi, Dr. Helmut Panke. And now I'm bored of scrolling through.