This is a statement that applies to terrorism, not to all-cause mortality. Statistically, it is a given; we can spend $Xm addressing Y different causes of death, watch how death rates respond, and have a pretty good idea which was the best investment. That's the whole reason the various departments have this dollar figure for a human life; they do this sort of analysis all the time.
Trying to apply that sort of economic analysis to a statistically nonexistent phenomenon like major terrorist attacks is utterly useless by comparison. It doesn't matter how accurate or inaccurate our assessment of the costs is, because we have no idea how likely they are to occur in the first place, and no framework to assess what the cost of preventing them would be.