like Goldman bring in these high-profile speakers is for their own marketing/public relations, both internal and external. It's a perk to their employees to get to hear the famous person speak, and it's a draw to potential clients to use a firm that does things like this, even if they themselves don't hear the speech. Clients are obviously important to companies, but so are very competent professional staff are as well, and these types of things are a lure to them. The speakers are there to engage, or entertain, or inform, not share top secret info with them or act as consultants.
Hillary is one of the best known individuals in our 7 billion population: given her experience, it should be a surprise to no one that many companies would be thrilled to have her as a speaker. If they were willing to pay a large amount of money for some famous male to speak to them, why shouldn't one of the world's most famous women be paid as much as him, if not more? Would it be wrong for Serena Williams to be paid more than Michael Phelps to give a speech? If they thought her stated fee was too much for them, they would have looked for someone else: it's not like they were being blackmailed by anyone. And as you said, she was a private citizen.
Leaving aside the bad actors who obsess about this merely as an attack on her, one of the difficulties that some people likely have about this is that they see these large dollars involved and can't conceive of someone paying that much money without an ulterior motive. But for the companies that hire such speakers, the money spent is actually not significant.
Regarding Cuban, here's a link to his speech. He's the first speaker, and goes for about 15 minutes. I jumped around it a little, but didn't hear a reference to the speeches.
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