The borders rail website says £294m
construction costs. That doesn't include a whole load of things like land purchase, consents, legals, design, feasibility, and so on. Usual rule of thumb is 20% for this. By coincidence that is £354m. The £355m is what NR have spent.
If by Summer 2016 you have a fully costed scheme to GRIP stage 3 (or equivalent - which is what funders require) with appropriate risk provision that says you can do it for less than £100m, I will indeed eat my hat. But I'm afraid you won't do that, as it requires extensive ground investigations, land referencing, structural assessments, environmental impact assessments, traffic impact assessments, constructability plans, consents plan with relevant statutory and non-statutory consultation conducted, assessed and concluded, alternatives considered, costed and discounted with evidence, and a full schedule with risk assessment. Plus reference designs suitable for approval in principle. Sorry if this is a bit extensive, but I have actually built railways (still am). It's not easy!
Railway construction costs are the costs of all construction on the railway. Mostly renewing and enhancing existing, but also building new. It's the same people, same techniques, same machinery, same materials. The only difference between new and existing is that there are fewer trains to worry about on new, but a lot more hassle from the neighbours. Most of whom
really don't want you to build a new railway in their back yards. And there is an awful lot of evidence out there that railway construction costs have increased.
Optimism bias for major railway schemes (classed as non standard Civil Engineering) has been 66% since 2002/3. See HM Treasury Green book. Projects can of course vary from this with a well developed Cost Risk Assessment, and/or if it is not being funded by taxpayers.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/191507/Optimism_bias.pdf
I must stress that I'm all for building new railways. But I want everyone to be realistic about the effort and costs involved in doing so. And realistic about who is paying, and why they should pay, especially when there are so many other calls on public funding.