I understand your point but part of the increase in cost is due to the length of time it has taked to overcome all the objections, both good and bad, and the resolution systems that deal with them. In a lot of other country (Good or bad) they would have been dismissed or upheld by a committee and the work would have started years ago. A little like the M4 relief which has been banging on for years. god knows how much it has cost, probably more than if they had just built the thing. I can recall 10/11 years ago listening to the then Chancellor Alistair Darling discussing HS2 on the phone, and then he was saying that it could run up to £50 million, and that was before all the delays.
All that said I think on balance it is better for the country as a whole that it is built rather than not built.