Pronoun trouble

The passage below is from a book, Justice, what’s the right thing to do.
In this text, the underlined pronoun “these” seems to represent “an amount representing the cost of dying early for the smoker and his family.”
If my guess is right, how can the plural pronoun “these” represent the singular “an amount”?

Thanks in advance.

http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1288608932

For a Benthamite, the smoking study does not embarrass utilitarian principles but simply misapplies them. A fuller cost-benefit analysis would add to the moral calculus an amount representing the cost of dying early for the smoker and his family, and would weigh these against the savings the smoker’s early death would provide the government.