I agree that its pocket change (well under $50M/year, 1/20th of the platform barrier cost). The overall congestion cost for the GTA is due to the impact of millions commuters on a daily basis (it's nearly all roads/highways, including most drivers and bus services, etc.). A TTC major subway outage does not occur daily and only impacts a hundred thousand people in a multi-hour outage.
Platform doors are worth doing but it's still a struggle to justify them from a financial perspective, even when indirect savings are considered.