The recent preliminary costings to complete this 35km line is about $750m(?) Yes much of the earth works is complete but still need to finish tunnel and two large bridges, one not even stated. However the O/H costs are quoted at around $140m for 35km of 1500VDC. That's $4m/km. Without O/H the project completion cost is ~$530m.
There have already been questions raised over this cost in the NSW section.
So before claiming the O/H costs are $2.5 to $4m/km. The new southern line in WA was built only a few years back for about $1.2B? and that's for 80km of dual bi-di trackage, tunnels, bridges and stations and I assume extra trains. So that's $15m/km. At $2.5m/linear track km that's $375m for the O/H. I'm still just not convinced that the O/H costs 1/4 of the greenfield costs.