But the roads and cars are used less, which means they require less maintenance. CO2 emission isn't the only pollution caused by cars either. People get cheaper transport options available to them.

All those other wins for society should be counted too, there may not even be a cost left to count towards CO2 reduction.

But most importantly, taking 3 billion in taxes and handing them out as subsidised pricing to some of the people isnt really a cost (to society as a whole), it's an income redistribution.