If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
You may value saving one life for $200, but maybe you're not willing to pay $400 to save two lives
This is what scope insensitivity is. The original paper calls it "purchase of moral satisfaction" -- the revealed preference in these experiments is for an internal state of moral satisfaction as opposed to the actual lives you're saving. Like hunger, the internal state is quickly satiated and so exhibits diminishing returns, but actual lives do not exhibit diminishing returns (in the relevant range, for humans, on reflection).
That would be an effective demonstration of scope insensitivity in an ideal scenario where money has a flat conversion to utility in that range for the individual in question. If $200 is in the subject's budget, but $400 is not, this may be entirely rational behavior. A donation which puts you into debt will have a much more dramatic effect on your own utility than one which leaves you solvent.