Subsea cable system costs are about 30k$ to 50k$ per kilometer. This is about the same cost as aerial construction, excluding the poles. You pay for the poles separately with yearly fees.
You also don't need roads to put up and service utility poles. It's just more convenient to put them by roads.
As an aside, you can and do have subsea cables with 216 or other high count strand numbers. The reason you use 12 strands or less in long haul systems is that it is cheaper to use fewer strands and higher bandwidth rates per strand than to build high count subsea cable systems.
You could build any terrestrial fiber system with just one strand, but it's cheaper and more convenient to use more strands since you are constantly bifurcating the network.
Source: I have built both subsea cables and terrestrial fiber networks.