A large part of the initial expense is right of way. Buying up/renting property or acquiring easements and defending the inevitable lawsuits is an ever increasing cost thats starting to rival the labor and material cost of the work. Maintenance is cheaper and will only continue to get cheaper relative to development costs unless there's a substantial change in the legislation governing that litigation.
There is a dc line from east of Portland to near LA, the pacific inter-tie. It was built as 2 wires in the 70s, carrying 1.4GW. Since then, over many changes, it was upgraded to 3.1GW. that involved equipment changes at both endpoints, but the lines and towers are still the same, 50 years later.
