Economists figure these things out by looking at how much people value their own lives (e.g. how much of a salary increase people expect for high risk jobs). The figures calculated are surprisingly low (IIRC ~$300,000 for an Australian in the late 90s). You need to do this kind of analysis to do cost/benefit analysis -- e.g. should we spend money on making a road safer or building a new hospital?
According to Wikipedia -- $6-7M per life in the US.