There's an argument that if someone agrees to a bad deal, that's their own fault. Where I think it becomes unethical is where there's a significant power imbalance that disadvantages one side.

Suppose I buy a painting from a flea market for $100, get it evaluated by a specialist, and then discover it's actually worth $100,000. In this example I have no inherent advantage over the seller; neither of us knew the value of the painting at the time it was sold.

Now suppose a famous TV antique dealer stumbled across that painting instead, and immediately realizes its true value. The seller recognizes the dealer, and the antique dealer offers to buy the painting for $25. The seller, trusting the antique dealer's judgement, agrees to the discount.

Would you say in both examples everyone acted ethically? This is a genuine question, as I can certainly see the argument that using the assets you possess to secure yourself the best deal possible is just business, and yet I would personally see the antique dealer in the second example as being exploitative.

When it comes to companies there's a similar disparity in power. An employee requires money to live, while someone founding or investing in a company often has enough of a financial safety net that they won't starve if the venture fails. Equally, any would-be billionaire is explicitly looking for employees who generate vastly more value than their cost. You don't get rich by paying people what they're worth; you get rich by underpaying them and pocketing the difference.

The other problem, and one you've touched on, is how do we assess the value of an individual employee? This is obviously not easy, and businesses also have no incentive to work it out or reveal that information to their employees even if they knew. On the contrary it benefits employers to keep their employees as much in the dark as possible.

Aside from the ethical problems there's a practical one. The very existence of billionaires implies that a significant number of people are undervaluing their work. It's a pricing problem that the market isn't solving, and is only getting worse.