Extortion is not "a perfectly permissible transaction" in any country developed enough to even think about trying to regulate biotech and medical research. But should a researcher be allowed to offer a poor person money or other needed medical treatments in exchange for allowing him/herself to be experimented on? Of course. Why should the government be allowed to set the poor person's priorities?
It is unethical to promote an environment where vulnerable people can and will be easily preyed upon by those who have the power to do so.
If you don't think that would happen, you have never seen a person who needs a liver transplant waiting for it.
If I understand you correctly, you do not think there should be any regulation on this at all. If so, a needy person who may have mouths to feed could be offered money to take part in a drug trial, which results in the painful prolonged death of that person who is released back to his family to die. His family will have no recourse. None. They will have the money which could be $20 or $200,000.
If you think what I have just outlined is completely within the rights of both the person getting paid and the people doing the paying, then you and I have a basic difference that is insurmountable.
If you agree that the situation outlined is intolerable, then we agree, is is just a matter of degree that we may disagree on.