The problem with these sterile arguments about car vs. rail is they hardly ever compare apples with apples.
When people talk about how much it costs them to go by car, they simply calculate the distance, work out how many miles they do per litre (or for some curious reason in UK, a gallon!), then multiply the litres by the cost per litre and - hey presto, that's the cost. Conveniently forgetting depreciation, wear and tear, servicing, insurance etc .etc.
HM Government agrees the cost of operating a car is 45p per mile. Try calculating car vs rail and availing yourself of the best advance fare offers and the gap narrows, and set against the hassles of the UK road network, I'd say it's at best 50 / 50 as to which is the best option travelling to airports like BHX, MAN and LGW - probably, in reality 60 / 40 in favour of rail.