A car journey of 45 miles at 40mpg would use 5.1 litres, or 10.2 litres return, or about £11.50 of fuel.
But you must also add in the 90 miles of car ownership cost, based on how many miles one drives per year vs. the cost of insurance, VED, MOT and sundry spares. That could quite easily be £500 per year, and anywhere from 3p to 15p per mile. A 90 mile drive to and from Glasgow in my last car would have cost an additional £15 because the operating costs were (quite) high compared with the (quite) low annual mileage.
The same trip by bike/train requires 20 miles of cycling plus 94 miles of train. Using my most expensive bike works out under present mileage as £14, plus the £10.70 train ticket. So er, hmm, it works out about the same as driving but takes twice as long. But hey, I'm all for not driving along the M8.
Taking a return train trip only costs the price of the ticket, so in some cases taking the train is actually cheaper than driving. The 'value for money' aspect for private transport always centres around using it as much as possible against the fixed or predictable costs. The trick is to use the more cost-minimal forms of transport where practicable, and to minimise the longer journeys for which public transport is appropriate and discouraging oneself from owning a car and then feeling duty bound to use it as much as possible 'to justify the cost'.
Edit: of course, Dave's quite right about maximising the number of people carried, where a car costs the same whether it has one person or five inside it, while every person needs to buy a train ticket.