There are much better solutions that target the actual large polluters. People living in suburbs is a small percentage of total global emissions, especially when you consider transportation of goods. Transporting goods across the oceans is a far larger percentage of emissions than people living in suburbs.
If the goal is to continue to survive as a species, there are much more effective strategies to implement.
Suburban living on average doubles the climate impact of the same number of people in a city. The OP notes correctly that this externality is not correctly priced in the current market.
How do you calculate the value of such an externality? Even if for argument's sake I were to just accept that it's double the climate impact on average, doubling a very small value is not really meaningful. The average American carbon footprint is roughly the same regardless.
You also haven't considered that as you move people into cities you need to scale up infrastructure in the city itself. Living spaces, transportation around the city, supply lines for supermarkets. Everything needs to scale. It's not black and white that moving everyone into a tightly packed city is going to make a meaningful difference to global emissions.
What would make a tremendous difference is turning the entire grid into a green smart grid powered by solar, wind, geothermal, etc. That would change things regardless of where people live, because the sources of energy are decentralized and can exist basically anywhere that natural resources like solar and wind exist.
You sort of can’t directly calculate externalities. That’s what makes them externalities.
The common answer is to not calculate it but to add a carbon tax. If you make gas more expensive, road taxes higher and non-renewable energy higher cost, the market actors will change their behavior.
As for the infrastructure cost, you pay that in either case and it’s more efficiently built the denser things are.
This isn’t a bottleneck style problem where removing the worst thing reorders the thing underneath it. We need to start targeting all of the inefficiencies and carbon emitters. The suburbs are worse for the environment plain & simple so we need to improve on that.
If the goal is to continue to survive as a species, there are much more effective strategies to implement.