I also think this is relatively orthogonal to cost; cost is nice because it helps keep the people you don't want in the train (crazy people with knives, people smoking weed in the train, people who yell at everyone on the train and make them feel unsafe, etc.), but it's also important to create at least a small barrier so that people don't 'waste' the transit system! In general, when things are completely free, they will be taken advantage of; even a very small tax / friction helps stop this. If the subway is completely free, there's no reason to not just sit on it all day (taking up space and making it worse for everyone). I think subsidizing the subway is net beneficial given that we subsidize cars and things already with road upkeep and such, but free is not what you want.