The struggle here is that you're talking about money earned after the fact and not including game theory. It would be a tough experiment to conduct, but say you spent $150 million to save $104. What if you didn't spend that $150M? Would you have an extra $40k in the bank? Or would the $104M in losses actually end up more like $1.2B because, slowly, everyone realised there was no reason to pay a fare?

I don't know what the case is here, but I imagine some economists have determined that $150M is enough to balance between actually getting people to ride the subway (increasing fare will eventually drive down revenue) and a substantial enough threat to prevent jumpers (no cops in the way means tons more jumpers).