That's kind of the heart of what's wrong with it. People generally agree, for example, that it was a good thing when we banned the practice of selling ones' self into indentured servitude. It is almost certainly true that indentured servitude was "worth the cost/risk" for some of the people who signed up, but we make it illegal anyway because it sets up a situation where people are likely to be exploited. Same goes for, for example, selling kidneys.