You shouldn't use X as your only variable as it sort of implies a fixed amount that is the same either way. However the systems are different and should have different costs.

I'm against free buses in general (though probably not in this particular case) because people want to get places. That money from bus fares should be put to running more buses (or building subways... lots of options, but more service). Nobody proposing free transit has ever proposed anywhere enough money that the city can run all needed services. They can maybe come up with enough money to continue current service, but every city in the world needs more transit service. You need service that can get you from where you are to where you want to be, when you want to go. Sometimes that place is farther away (but not unreasonably far) and you need high speed express service. Sometimes you are running late and miss the bus/train - is the next bus/train coming soon enough that you are not unreasonably late? Sometimes you need to be out late, is service still running? Sometimes you want to do something on a weekend. More service is needed everywhere and that lack of service is the primary reason people buy cars which are ready when people want to go (they claim otherwise but when I dig down it comes down to service)