Much of the infrastructure cost is boring stuff like earthworks and retaining walls and moving existing services that are in the way. The shift to in-cab signalling saves/will save a bunch of money on lineside equipment maintenance (because you don't need it).

I was thinking about your comment and one way to save considerable infrastructure costs is more level crossing and less overpasses/bridges, but train vs pedestrian or train vs car are common and messy. Plus level crossings wreck traffic flow on the surrounding road network.