I haven't visited Paris in maybe a decade+, but is this going to be much of a financial issue for locals? Do most commuters use monthly rail passes, or do they buy single use fares?
To me, it makes sense to increase costs, because there will likely be an extreme surge of people using the system, requiring more resources and employees during this time period. I'm not sure of the economics of it all, but it's possible that the usual fares, even with the increased number of buyers, might not offset the increased costs.