From the Freakonomics blog: "FDA prohibits any gifts to blood donors in excess of $25 in cumulative value".
Various articles give different amounts for the price per pint that hospitals pay, but it looks like it's in the range of $125 in most cases.
Basically that means that the FDA thinks that putting that limit on blood donations won't reduce the amount of blood donation in critical way that results in people dying as a result.
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