Driving distance using a car is 15 miles leaving at 8:10am taking roughly 25-30 minutes in the morning, 30-45 minutes in the afternoon. (Leaving around 5:30pm.)
The bus requires driving my car 3 miles to get to the nearest stop in the morning. Once on the bus, one transfer in downtown Nashville is required, adding on an extra 5 miles for a total of 20 miles one way. I would have to be at the stop at 6:30am in order to get to my final stop at 8:15am then add another 10 minute walk after that, putting me at work at my normal time of 8:30am.
To get home I would have to leave work at 4:00pm to catch the last run back to La Vergne arriving at my final stop at 5:45pm. This results in a 6.5 hour work day.
So, riding the bus results in a much longer commute, and assuming my calculations are correct, fare costs nearly twice as much compared to driving. $3.50/day vs. $6.13/day. That's using a 31 day pass for the route in Nashville, plus the special fare to ride the Murfreesboro/Nashville line(96X) from La Vergne into downtown. The 96X fare isn't included in the 31 day pass.