Blood has a low cost to the donor, so the "feel good" intrinsic reward is high enough to keep the supply going. Adding a price will just make it more expensive for poor people who need transfusions.
Because kidneys are so expensive to donate, you can't rely on people's goodwill, so both poor people and rich people miss out. Of course, you would have to ensure that they wouldn't be sold to clear debts.
Blood has a low cost to the donor, so the "feel good" intrinsic reward is high enough to keep the supply going. Adding a price will just make it more expensive for poor people who need transfusions.
Because kidneys are so expensive to donate, you can't rely on people's goodwill, so both poor people and rich people miss out. Of course, you would have to ensure that they wouldn't be sold to clear debts.