Pay today or pay tomorrow. I don't think it is an outlandish thought IF $100M/mile tunnels are achieved that the TCO for an elevated roadway will be higher. Elevated roadways have to be replaced eventually. Tunnels are more durable, look at the Holland Tunnel for example, 89 years old and no need for replacement in sight. And it's $48M price tag would be $3.3B today for 1.62 miles.

Elevated roads require surface space. A tunnel requires a small entry-point.

Well, given that both tunnels and elevated roads are still being built, the most expensive likely varies depending on the situation. If Musk really reduces the cost of tunneling by 10x, that calculus will shift in the direction of tunneling, obviously.

Dirt roads are extremely cheap and have been around forever. That doesn't make them the best choice in a modern society.