This sounds like a job for the
marginal cost per mile.Basically, since you're driving 10 miles each way anyways, there are a lot of fixed car costs (like the cost of the car, some of the wear and tear, insurance, tc) that you are going to incur regardless of whether you drive part of the way or the whole way. In that thread, I estimated the marginal cost of a mile to be ~$.25, about half of MMM's (and the IRS's) quoted ~$.55.
Using MMM's math, 15 miles x 2 = 30 *.55 = $16.50 * 20 days/mo = $330 - $66 for the bus pass = $264.
Using my math, 15 miles x 2 = 30 * .25 = $7.50 * 20 days/mo = $150 - $66 for the bus pass = $84.
Is $84 - $264 worth an hour of your day each day? Up to you. For me, $84/20 = $4.20 an hour, worth it, and even using MMM math of $264/20 = $13.20 an hour, still worth it.
But that value judgement is up to you. You can also tweak my assumptions based on your car (mileage, etc).