That isn't a whole lot more than the cost per kilometer to construct urban roads with a comparable passenger throughput, though. And construction cost alone isn't exactly a fair price comparison because users of urban roads tend to need to supply their own vehicles, at considerable cost to themselves, and more money also has to be spent to provide space to store those vehicles when not in use.

And you could argue that roads are actually even more expensive to invest in because they tend to create a feedback cycle that promotes even more spending on road construction. Because as more people use roads, you need to widen them to increase their carrying capacity, and because road-oriented infrastructure encourages sprawl that pushes people toward choosing driving, and spending more time on the roads, which increases the need for more roads to be built.

A nice case study here is Chicago versus Detroit. Proportionally speaking, they had similar and approximately contemporaneous population growth and loss trends over the 20th century. But that trend didn't cause the same financial catastrophe for Chicago that it did for Detroit, and one argument for why this is is that, since Chicago retained a denser, more public transit oriented urban plan, its infrastructure costs relative to population stayed lower, which in turn meant that it was able to weather a lot more population decline without its economy shrinking too much to be able to affordably maintain its infrastructure, including transportation infrastructure.