So the gas cost of an exit is about ~80-90k gas, it's hard to get exact estimates w/o additional information. If we could somehow take up the entire block gas limit (~8m currently), then we'd get about 4m exits completed. Each exit would cost ~$0.30 @ current gas prices.
That's without challenges and whatever else. We can't assume that every block will be full, so it might be better to only assume 25% block usage ~= 1m exits. Of course that's if everyone instantly sees the block withholding and starts exiting right away. I don't think we'd actually get 1m exits submitted.