Its a question that is often hard to answer – Unless you are taking about a bureaucratic government run health care system. According to the WSJ, when looking at how the British National Institute for Health and Clinical Excellence (NICE) decides what treatments are reasonable (can you say “Comparative Effectiveness”?) its pretty straightforward,
The NICE board even has a mathematical formula for doing so, based on a "quality adjusted life year." While the guidelines are complex, NICE currently holds that, except in unusual cases, Britain cannot afford to spend more than about $22,000 to extend a life by six months. Why $22,000? It seems to be arbitrary, calculated mainly based on how much the government wants to spend on health care. That figure has remained fairly constant since NICE was established and doesn't adjust for either overall or medical inflation.
So the NICE board decides whether you can live another 6 months. And their main decision point is “does it cost more than $22K?”