Yes, transit costs a total X. In the car regime everyone puts in a small amount towards public transit and roads and richer folk put high amounts towards cars, totalling X. In the public transit regime everyone puts in a medium amount towards roads and public transit, and a negligible amount of tradespeople and construction companies buy their work vehicles, totalling X.

You can choose the car regime if you want, the US does, but: 1- public transit is lower quality due to higher income brackets choosing cars. 2- everyone is screwed by the cars' negative externalities (noise and air pollution mainly). 3- lower income brackets are screwed by the traffic generated by the higher guys (50 minimum wage workers occupying the same lane space as 3 SUV-driving middle managers). Also you have to remember how much the mortality increases in higher car traffic areas, so that X figure isn't really true

You shouldn't use X as your only variable as it sort of implies a fixed amount that is the same either way. However the systems are different and should have different costs.