|April 23rd, 2018
On Friday evenings when going by myself from work I've taken the train. There's an express that leaves South Station at 6:10pm and gets into Mansfield at 6:45pm ($10). From there's it's either a 20min walk or calling the festival shuttle and asking for them to pick you up. Door to door, including the Red Line to South Station, it's about 1hr15min, faster than driving there in rush hour traffic. I've been able to find a ride back each time with a friend.
On the other hand, when going from home instead of work, and when not comparing with rush hour traffic, public transit isn't really competitive. (Plus right now the Red Line is running shuttles on weekends, which slows things down even more.)
Some years we've been able to carpool with others, usually family. Now that there are four of us this is much less practical, since it would need to be someone who would otherwise be driving alone.
Last year we rented a car for Saturday and Sunday, which is what I think we'll be doing most years. It was $64, plus $10 gas, plus about 15min of walking to pick it up and return it.
This year it happened to work out that a musician friend was flying out of Logan and wanting somewhere to leave their car. Our driveway is generally empty, so it's easy for us, and they're ok with us driving it some while they're away. I'm planning to reimburse them for depreciation which would be about $16 (155mi)  and gas was about $12, so say $28 for the two days.
I've thought about whether it would be worth it trying to organize a shuttle from Porter Square. Probably charter a bus, with one run to the festival Saturday morning (10am? arrival) and another run to return that evening (10pm? departure). If we went low end (school bus) and filled the bus it would probably be something like $20/person. Not really competitive with renting cars.
 Kelly Blue Book estimates at 4¢/mile for this car with this mileage, but that is assuming that the condition of the car doesn't change. While the condition is unlikely to change, every mile there's a chance that something goes wrongn and means you need expensive repairs. I'm figuring 10¢/mile, which is mostly just that it's a nice round number.