Yeah that's already factored in as savings and thus deducted from the costs:

The cost of eliminating fares from all city buses would likely be north of $700 million – an MTA analysis from 2022 put the cost for fiscal year 2026 at $778 million, Mamdani’s campaign said. A separate analysis on free local bus service from the city Independent Budget Office incorporated savings that fare-free bus rides would produce, including on fare enforcement and collection costs, totaling $33 million per year. They found the total cost would be $652 million.

I already deducted an extra $52 million for a good-looking number. Whatever the independent analysis didn't think of is not likely to go beyond this $52 million.