This comment confuses me.
The point of the excerpt you quote has nothing to do with income at all; the point is that (for example) if I have $100 budgeted for charity work, and I'm willing to spend $50 of that to save 2,000 birds, then I ought to be willing to spend $75 of that to save 10,000 birds, because 2000/50 > 10000/75. But in fact many people are not.
Of course, the original point depends on the assumption that the value of N birds scales at least somewhat linearly. If I've concluded that 2000 is an optimal breeding population and I'm building an arcology to save animals from an impending environmental collapse, I might well be willing to spend a lot to save 2,000 birds and not much more to save 20,000 for entirely sound reasons.
If I budgeted $100 for charity work and I decided saving birds was the best use of my money then I would just give the whole hundred. If I later hear more birds need saving, I will feel bad. But I won't give more.
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