Although I found this page when searching Massachusetts-specific options, the calculator should work for anybody who drives their commute. Enter in all the details (distance, mileage, cost of gas) as well as a few you might not have thought of (insurance, maintenance, both of which they will default for you based on national average), and learn how much you're spending on your commute.
I'm spending somewhere near $450/month according to this thing, and that's just to get back and forth to the train station. How about you?
(I see that it's the insurance cost, something that most folks won't really be able to calculate off the top of their head, which is the biggest factor.)