I say: Give numbers for specific questions. If we want to build the Gothard Tunnel, for example (actual construction started 1871) it would be ok to give us an estimate of the costs in money units, with the usual caveat that actual costs may be higher. Or if I want to recruit 10,000 soldiers, I would have a fairly good idea of what it'll cost me, so I should be able to ask Perth.
Just roll a dice to determine how accurate the cost prevision is. There can always be unexpected circumstances, or disasters, that make things more expensive (or even cheaper).
