Yes - it's impossible to reliably and accurately estimate (which is why it's an estimate). But you've got to have some form of figure, both of cost and revenue

Yes - it's impossible to reliably and accurately estimate (which is why it's an estimate). But you've got to have some form of figure, both of cost and revenue - even if it's just back of a beermat guesswork based on a vague hunch - to make any form of sensible trade-off judgement. I do a fair amount of work around business cases for new projects, and simply forcing the proposer to go through an exercise of answering "how do you think this project will increase revenue/reduce costs/whatever, and by how much?" helps weed out ideas that don't really have legs or haven't been properly thought through, as does having a first stab at "What's involved in delivering this? Is this a 2 week project or a 6 month one?"