Most of the car costs are "sunk" costs, i..e they've already happened, so aren't part of the decision making process. It's only marginal costs that matter when decision making, i.e. the "extra" costs, such as fuel, slight increase in depreciation, slight increase in repairs/renewals, i.e. tyres and brakes would need replacing a bit sooner, slight increase in insurance if based on mileage, etc.
Many people will need a car for other purposes, especially those who don't live/work/shop near good public transport, so for them, it's basically a simple question which mode is cheaper for an extra journey. Of course, for those who don't have a car for other purposes, then it's a much bigger question whether or not to buy a car for specific journeys and that's when you have to compare total car costs inc the fixed/sunk costs. But in the latter case, you have to compare car costs against ALL potential public transport costs, as once you've got a car, you'll use less public transport, so it's no longer just comparing one journey mode with another for specific journeys.
I spent years studying and then working as a management accountant - it's fascinating work doing all kinds of "what if" scenarios, and it really hammers home what information is needed to enable proper decision making processes, inc all the different permutations etc.
