Organ transplantation costs tens to hundreds of thousands of dollars, including the costs of doctors, hospital time for recovery, drugs, etc. Paying less than $5,000 for a legal kidney (at prices in Iran), or even $25,000 in rich countries with more expensive surgical staff, makes for a trivial portion of the overall cost of the operation. Why make the whole operation illegal (making it hard to advertise, recruit employees and customers, avoid imprisonment, etc) by not using voluntary donors to reduce costs by a few percent?

Consider that the surgeons make up a much larger portion of the cost. Should we fear that legalizing organ transplantation will drive a criminal industry kidnapping and enslaving surgeons to perform organ transplants? We don't see such an industry for coronary bypass operations, so we shouldn't expect it for organs.