The fare has to be low enough that poor people can afford it because they have to be able to use mass transit to get to work. Even the homeless need to be able

The fare has to be low enough that poor people can afford it because they have to be able to use mass transit to get to work. Even the homeless need to be able to use it so they can e.g. get to a job interview. And a fare is not going to be a deterrent to juvenile delinquents because they're the ones jumping the turnstiles. But if you're not actually excluding the deplorables then the fare is just a regressive tax and a deterrent to mass transit use for everyone. The real problem is that you have unsheltered people without taking effective action against it and then the subway is a form of shelter. If you want to help them then you fix the zoning so more housing can be built, provide shelters, etc. If you want to be a cold bastard then you give them a one way ticket to a lower density city. Charging 10% of the cost of the transit system as a fare, which costs about as much to collect as it generates if not more, is neither of those things and counterproductive.