Okay, but what if a person sincerely does not want to participate in a medical trial? But then when he sees the dollar figure, he thinks "well, shit, I hate doing this, but I can't afford to turn that much money down."

Same reason we don't allow the purchase of kidneys, and the same reason we think it's unethical for a millionaire with a tophat and a monocle to offer two homeless guys a wad of money to the winner if they fight each other. Sure, it's an adult making a decision of their own free will- but it's obviously unethical.