Just to be clear, I think that in jkaufman's argument, the $400 is not the opportunity cost of undergoing surgery, but rather an estimate of the amount of money AMF would require to achieve comparable benefit as donating a kidney.

I agree that if you had to forego income during surgery or incur other costs, then it would be reasonable to add those onto the other side of the ledger as you propose. As I mentioned above, though, I think that's typically not the case; medical leave policies are often quite generous in these cases, and public funds are available for incurred costs.