In the comments below, Robin Hanson writes:
I can't imagine what you are thinking in saying economists have no competence to say what is a cost or benefit. That seems to me to be one of the things we know best. And the market interest rate clearly gives the opportunity cost of resources spend in the future. You complain about cost-benefit analysis, but seem to have nothing to offer in its place.
I will help you imagine! I am thinking that the meanings of “cost” and “benefit” are either contested, due to diversity in reasonable evaluative standards, or their meanings are stipulated for technical purposes.
If the meaning is plural and contested, economists have no special comptence to decide between the different evaluative standards underlying different meanings of “cost” and “benefit”. Economists are people, and people can make arguments and exchange reasons for an against various evaluative standards, but economists do this as people with a conception of value, not as economists.
If the meanings are stipulated to make a kind of a toy analysis tractable or determinate, that's fine. But then the toy analysis has force only for those who already accept the stipulated meaning of the terms.
I have to get on the plane, but when I get home, I'll say something about policy evaluation that takes evaluative pluralism seriously.