How far away is work, what type of Metcard would you be using (zones?), and do you drive a Prius?
I'm assuming you're ruling out other costly factors such as time spent waiting for public transportation and personal preferences such as how much being squished against other people/not having a seat shits you. There's also the weather. During the winter influenza season you could avoid infection from commuters by travelling in your protective shell of isolation, and then get sick from a co-worker instead - costing you money to get some drugs, feel like there's an elephant lodged inside you, can't turn up to work etc. If you drive in the summer heat you'd be tempted to crank up the air con a notch, which eats more energy. Also in that weather, public transport wouldn't be the place of choice unless you don't actually mind nasty BO and all the other unmentioned nasties that could burn your imagination.
Having said that, don't drive, get off my road, I have feelings of selfishness too.