I invite you to read https://www.researchgate.net/profile/Carl-Elliott/publicatio...

If you genuinely think the medical and pharma industry don't exploit homeless and other marginalized populations out the wazoo as lab rats. They also only stopped doing it to prisioners when it was made illegal, because that's what they did before. Of course using them has a lot of practical problems, as you outline, other than the much bigger ethical issue.

You managed to miss the whole point of the comment though.

Which is, how is it possible that a multibillion dollar industry, exploiting both the test subjects AS WELL as the researchers in the form of practically free PhD candidates can still claim it's one of the most dificult / expensive endeavours? Where does the money go? When you look, it goes to the bureocrats and to the "bio-investors".