A recent report by the Associated Press attempted to answer this question by looking at the costs of prescription drugs. That report concluded the cost of extending the life of terminal cancer patients by just one year can be as high as $800,000. That's how much Medicare and insurance companies are willing to pay for drugs that keep cancer patients alive. A more commonly-seen figure is $50–100,000, which is still quite a bit of money to most Americans.
This question — how much is a year of life worth? — is a touchy subject in the practice of law. Every single one of my clients who have been injured, or who lost a family member, wouldn't take any amount of money if they could just put everything back to the way it was before the wrongful occurrence. That can't happen, so every case involving personal injury or death requires a jury to place a dollar amount on the value of human life. That's a hard thing to do, but our civil justice system has no other way of correcting wrongs. There is no "eye for an eye, tooth for a tooth" justice in civil cases. Making someone pay for their mistakes comes down to just that: money is the only remedy for physical harm permitted in our civil courts.