Separately from the fact that I don't think $400 gets as much benefit as you seem to, I don't think the idea that you would be willing to pay $400 to avoid a nephrectomy is a good reason not to donate a kidney, at least without the additional stipulation that you're not willing to donate $400 more. Imagine a case where an individual believes that donating a kidney is worthwhile for benefits to others, but that they would happily pay $400 to not undergo the surgery (this may be your case, I think it would be mine, too). He pays the $400, and then face the question again. He still has both kidneys, can still benefit someone else by the same amount as before, but he has $400 less, so then he decides again whether to donate a kidney or donate $400. (...induction...) Eventually, he either donates a kidney or decides that enough has been asked of him. Another, perhaps more straightforward, way to put this is to say that the fact that there's some amount of money that achieves equivalent benefit to donating a kidney only tells you not to donate a kidney if you're sure you're not willing to give more money; the implication could just as easily be that you need to give a lot more money. I'm actually not sure what the amount of money (i.e. the $400) has to do with it in your framework, since for any given amount of money the repeated question means that you eventually reach your limit in willingness to pay to avoid surgery.

If the opportunity cost of donating a kidney is $400, because of the opportunity costs of not being productive during the surgery and recovery, then you are effectively donating $400 in addition to your kidney to the cause of giving someone else a kidney. It seems reasonable that the $400 would come out of your charity budget, and you should consider if another charitable cause could use it more effectively.