It is a real problem to closely define these external cost.   I don't take them as pure fact on cost, because there are so many variable in cost that all numbers have a lot of speculation.    

But logically, those items that the writer brings us as external cost, are most certainly good things to consider that have a cost.    We can either ignore them because they are imprecise, or accept them as factors that should be considered.

Sometime a tactic of persuasion is to obfuscate the issue with detail. (baffle with BS)    The writer does some of this by asking us to have a high level of trust in the numbers.  The opposition does this as well, by demanding only perfect clarity when perfect clarity it impossible.

If you've worked in a team, you know that not all problems have immediate answers.   That doesn't stop good teams from addressing a problem.  Good leaders are optimistic that unseen challenges can be met.     Great challenges come with risk, uncertainly should not mean retreat.