A basic road asphalt road isn't even that cheap.
True but that's only if you use first world prices with pampered workers.
Even in Africa they use the same costs; though most of it gets eaten up by foreign firms or corrupt procurement practices.
If we build everything in-house and use Somali labourers who are content with just a bundle of khat then we could easily get the price close to $100k/km (min 500 km).
Remember it doesn't need to be complicated... Just efficiently move goods and we're good to go.
Stop using historical or western projects as an example to costs out these projects.
We have to innovate and do things in our own way if we ever plan to leap frog and catch up to others.