Here's how I look at it. The money doesn't even enter into the equation; it's simply an unintended bonus.

I am severely overweight. I'm a lot less overweight than I used to be, but I'm still 150+ pounds above where I want to be. I need to lose weight. In my life I need to get from point A to point B frequently. I can choose to do so with an automobile, or I can choose to do so with a bicycle.

When A and B are my workplace and I chose to use my car, the variables are

Time: 40-50 minutes (and occasionally 2 hours if traffic is bad, but that's an outlier)
Energy expended: Virtually zero

On my bicycle they become:

Time: 70-100 minutes (and occasionally 2 hours if I have a flat, but that's an outlier)
Energy expended: 1000-1200 calories

Now, prior to my biking-to-work phase, I was exercising 120 minutes a day on average, trying to lose weight. So if we added up the time I spent cycling for fun, and the time I spent driving to work we get:

120 + 40*2 to 120 + 50*2, or 200-220 minutes a day.

Now, if eliminate the drive and cycle to work, this figure becomes:

70*2 to 100*2, or 140-200 minutes a day.

So by cycling to work, I actually have anywhere from 20 to 60 minutes extra per day, to do whatever I want. I get to exercise more, and save time at the same exact time. It's really a no-lose scenario for me.

As long as I need to lose weight, and this condition will remain true for at least a few years, bicycling saves me time. It literally makes no sense to drive unless I simply need to be somewhere faster than I can cycle there. Now here's the best part. I would have spent money on my bicycle regardless of whether I commuted or not, because I would be riding the bicycle anyway! So yeah, there's a few things I purchased for commuting, like the lights for commuting at night (which I now use for cycling at night even when not commuting!), but by-and-large, almost none of the money I've put into my bicycle is only used for commuting. So I'm saving money simply by not driving, no matter what, and all that money is a bonus, in addition to the time-and-health benefits I gain regardless.

I really can't stress how much of a win-win scenario this is.