Good points. It's obvious that the potential for ethical problems in such a situation is huge.
I've thought for similar questions that a required baseline would be to at least have a society that provides a viable minimum social safety net, as well as free healthcare. When external conditions compel people to risk their health for survival, it is impossible to ensure sound ethics.
The point about ensuring that science and scientists can be trusted in the future is also an excellent point.
Bill Gates or Jeff Bezos will never risk their life for "only" a million bucks. Find a depressed + suicidal dude somewhere else, and they may be personally willing to risk their life for just $100.
Making an industry of people who are willing to harm themselves for money is... probably not the business we should be encouraging. And that's assuming everyone is "playing nice". At a minimum, it means that we'll be inclined to start experimenting upon the poor and depressed (and then shedding away moral qualms because they signed a piece of paper).
Historically speaking in the USA, we have legacies like the Tuskegee Syphilis Study, where African Americans were chosen as the control group and lied to about getting a Syphilis treatment: leading to a generation+ long distrust in science.
Yeah yeah, we can't "assume" scientists will be racist jackasses today. But... we still have to account for what happened in our history and why certain testing methodologies have become taboo.