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.
The whole issue would be resolved if, in order to receive a donated organ, adults would have to already be registered as donors (prior to the diagnosis).
"In order to get, you have to be willing to give" is a simple protocol and would easily resolve the issues surrounding organ transplants.
People make their own decisions to take risks every day. Medical research and organ donation shouldn't be any different. The government doesn't own the bodies of individual citizens. In a free country, each of us owns our own body and must have the final say as to what we do with it, whether or not compensation is involved.