I don't know how much roads cost to build. $5 million per mile? No doubt, it depends on the road.
But it's clear Taipei City has spent a bunch of money on the riverside paths around the Tamsui river. They're in fantastic shape.
This is surely one of Taipei's marquee loops. It runs about 29km and takes you through a range of scenery. There's lots to see and plenty of places to stop and soak up the view.
But if you want, you can do the whole thing in one go without stopping at all.