On one hand, it might be the case that the actions which are most cost-effective at doing good actually do very little good, but are also very cheap (e.g. see this post by Hanson). Alternatively, maybe the most cost-effective actions are absurdly expensive, so that knowing what they are doesn’t help us.
I think the first argument can be rescued by including search costs in the “cost” definition. I agree that the second one cannot be, and is a serious issue with this phrasing.
I think the first argument can be rescued by including search costs in the “cost” definition. I agree that the second one cannot be, and is a serious issue with this phrasing.