'The construction industry needs to be better at sharing if it's going to learn and improve'
The best way to estimate is really to understand the context behind the costs. If you know why something costs what it does, then you can share that understanding with others through your estimates. When we're all on the same page, everyone has a better chance of getting the right thing done at the right time. Furthermore, the more we can share about data, the better and smarter we will be. In fact, some believe that data should be shared to the maximum degree possible as it can help us all become better at what we do.
That being said, a project's scope is the sum of all work involved in a project. It represents the implied or explicit commitments made by all parties. Understanding what constitutes "scope" at the start of a project, and ensuring that this is communicated effectively to everyone involved, can make all the difference between success and failure at the end of a project.