If 10.8% of people would be less likely to donate, and 68.2% of people would be unaffected.... doesn't that mean 21% of people would be more likely to donate kidneys? If so, the evidence would suggest paying for kidneys would generate enough new donations to more than offset the number of donations that would be lost.

I can understand the metaphysical argument, though I'm not bothered by the commodification of body parts (isn't that what the exchange of labor is, after all?). But unless you got more numbers you failed to divulge, it seems like you should stick to the metaphysics... because the math isn't doing you many favors.