Well, it's money or time, isn't it? Just for example, I could take the light rail to work every day and save a fair amount of money (I've already crunched the numbers.)
What it would
cost me, though, is probably 1 1/2 to 2 hours per day in commuting as well as the flexibility of leaving to and from work on my own time rather than on RTD's time. Ultimately like most of us (judging from traffic, anyway) I've determined that my time is worth the money it costs.
A few years back the wife and I took a weekend trip to Glenwood Springs. We decided it would be fun to take the train rather than drive. And it was fun, that's a gorgeous train ride. But the train left Denver at 8:00 AM and we got into Glenwood around 2:30, 6 1/2 hours later.
Driving would have taken 3 hours AND we would have had a car when we were there, as opposed to having to rely on the hotel to pick us up from the train station and being on foot to go to dinner and such (which, in a town as small as Glenwood Springs, isn't that bad.) And of course that doesn't even take into account the fact that we had to get a ride to the light rail station and leave our house by 6am. So what would have been 3 - 3.5 hours in the car became a full day of travel.
It's always money or time.
EDITED TO ADD: We also didn't save any money taking the train, quite the contrary. I want to say it was at least $75 for each of us and of course we had to buy two tickets. Even at $4.00/gallon and driving my gas guzzling Suburban at 15 MPG (IOW absolute worst-case scenarios cost wise) we would have used 24 gallons of gas to drive 360 miles (180 miles each way) which works out to $96 travel costs and obviously that's the cost for both of us and all the crap we can carry.
I guess my point with all of this is that while it's true that cars cost money, what they save us in terms of time, convenience and freedom of mobility is what makes it worth it.