I have had many conversations about this with my brother-in-law who is a civil engineer and he always reminds me of two variables -
The first is that a massive cost over-run is guaranteed simply because a project of this magnitude will uncover unforeseen problems on a daily basis that could not have been allowed for and factored into the original costings, so the budget will bust as a matter of course.
The second is that a project of this magnitude and cost simply cannot be stopped once it has got underway, the lost revenues in terms of initial research and preparation mean that once it has started, the project will always be seen through to the end because to do otherwise would mean a lost in revenue that will dwarf even the predicted overspend as the project rolls out.
That said, I feel that the destruction of housing and countryside remains at an unacceptable and frankly immoraL level, only to increase as time goes on.