In this case it seems like the marginal value of blood donation should be roughly what the organizations like the red cross are willing to pay to get additional blood donations.
You could look at how often patients get less blood because of supply issues.
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.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.