It costs money to draw, test, process, transport and infuse blood. The British healthcare system is single-payer and incredibly efficient, but an NHS hospital pays the NHS blood service £124.46 ($168.59) for a unit of red blood cells.

Paying donors for blood was a major contributory factor in the tainted blood scandal of the late 1980s and early 1990s; there's an unpleasantly large overlap in the Venn diagram of "people who are desperate for cash" and "intravenous drug users".