The FAA uses a cost of $2.7 million USD for a human life. That is, if the cost of a "Fix" for a plane's defect exceeds $2.7M per person estimated to be saved by fixing the defect, it's deemed too expensive, and the fix is not made mandatory....
http://fast.faa.gov/Riskmgmt/Secrisk...cs/00-11-a.doc