i disagree with the kind of analysis you're doing here. Just because a difference is a small percentage of the maximal possible difference does not mean it is a small difference. Suppose you have three mutually exclusive actions available to you: A - play video games all day, B - put in a small amount of effort that will ensure that malaria is eliminated, C - put in a moderate amount of effort that will ensure that everyone in the world lives forever without sickness or deprivation. As a percentage of what you could accomplish by doing C, the difference between doing A and doing B is negligible. But it seems absurd to say that the difference between doing A and doing B is so small that it doesn't really matter. Sure, it's small as a percentage, but it's still a massive difference.
Of course, it still might be the case (in fact, it probably is the case) that in your particular example, the cost of intervention outweighs its benefits. But I have already granted that this is often the case. If your example was meant to provide a general argument against any government regulation of resource usage, I don't see how it works.
Instead of discussing hypotheticals, why not talk about the actual policy under consideration: Do you think that the cost of a mandatory organ donation policy plausibly outweighs its benefits? If you don't, do you have other non-consequentialist grounds for opposing the policy?
It doesn't matter because you could add a rediculously tiny tax to make up the difference.
Also, I think it's generally a good idea to give taxes and subsidies to get rid of externalities. If this is done correctly, it makes no difference at all how you spend your money.
Or is the convention against discussing politics here silly?
I propose a test. I'm going to try to lay down some rules on voting on comments for the test here (not that I can force anybody to abide by them):
1.) Top-level comments should introduce arguments (or ridicule me and/or this test); responses should be responses to those arguments.
2.) Upvote and downvote based on whether or not you find an argument convincing in the context in which it was raised. This means if it's a good argument against the argument it is responding to, not whether or not there's a good/obvious counterargument to it; if you have a good counterargument, raise it. If it's a convincing argument, and the counterargument is also convincing, upvote both. If both arguments are unconvincing, downvote both.
3.) Try not to downvote particular comments excessively, if they're legitimate lines of argument. A faulty line of argument provides opportunity for rebuttal, and so for our test has value even then; that is, I want some faulty lines of argument here. If you disagree, please downvote me, instead of the faulty comments, because this post is what you want less of, not those comments. This necessarily implies, for balance, that we not excessively upvote comments. I'd suggest fairly arbitrary limits of 3/-3?
Edit: 4.) A single argument per comment would be ideal; as MixedNuts points out here, it's otherwise hard to distinguish between one good and one bad argument, which makes the upvoting/downvoting difficult to evaluate. (My apologies about missing this, folks.)
I'm going to try really hard not to get personally involved, except to lay down a leading comment posing an argument against abortion, a position I don't hold, for the record. The core of the argument isn't disingenuous, and I hold that this argument is true, it just doesn't lead to my opposing abortion. I do not hold the moral axiom by which I extend the basic argument to argue against abortion, however; I'm playing the devil's advocate to try to help me from getting sucked into the argument while providing an initial point of discussion.
Which leads me to the next point: If you see a hole in an argument, even if it's an argument for a perspective you agree with, poke through it. The goal is to see whether we can have a constructive political argument here.
The fact that this is a test, and known to be a test, means this isn't a blind study. Uh, try to act as if you're not being tested?
After it's gone on a little while, if this post hasn't been hopelessly downvoted and ridiculed (and thus the premise and test discarded as undesirable to begin with), we can put up a poll to see whether people found the political debates helpful, not helpful, and so on.