A friend of a friend runs a pipeline construction company (think the people that would actually build Keystone XL or the likes) and he has a team of estimators that run all the numbers on a given project and come up with a budget. He then double checks their numbers by chartering a helicopter and flying the route with an an old construction foreman, who basically eyeballs the thing and and does some mental math. Apparently it was uncanny how often the two estimates were within 10% of each other, but that the old guy eyeballing the thing had saved him on a number of occasions by recognizing issues/costs that the estimators had missed...

After spending his entire career in construction, one of the last jobs my dad had before retiring was in preparing bids for proposals. The company wanted people with lots of experience in the bidding process. Not just on-budget, on-schedule, but people that had been through over-budget, behind-schedule so that they knew what to be aware of as potential pit falls.

When I was in college one of the big promises of BIM was "we'll have information rich models that will make estimating easier and more accurate!"

I'm not directly involved in that process in-industry (ended up in light fixture manufacturing), but my impression is that BIM models are usually good enough to kick out a set of drawings, but you wouldn't want to count on any information that you pull out of it electronically being accurate.

So you still have "contractor to verify quantity" notes on everything, and presumably someone going through the drawing figuring out how many "Type F 2x4' troffers" and how many faucets and how many of every other little detail that will take money or time.

It's a pretty intensive process, but given the amounts of money at stake if you screw it up, hugely important to get right.