These are immensely complicated problems by the time you get into them. “High”, “low”, and “most likely” is a way of doing it, or saying what the likely standard deviation is going to be from it, etc. It depends on the problems and on the person looking at them as to how they do it.
But there are ways of narrowing the uncertainties, which is what we really want here, so that we have more confidence when we're looking at a bill that we understand the cost implications, so we're not just comparing apples with oranges, but we're comparing apples at 50¢ each with oranges at $2 each, and that sort of thing.