To put some numbers on it, US carriers are canceling roughly 280 flts a day right now. I don't know how many depatures a day airlines had around 9/11, but I would guess it was a few tens of thousands. (Flight aware has tracked roughly 39,000 arrivals in us airspace in the last 24 hours. Granted some of those were coporate/private aircraft etc.)

European carriers normally operate about 20,000 depatures a day, and right now are only operating about 4000 of those. They hope to be back up in the 10 - 12,0000 range tomorrow.

So, its probably comparable to 9/11 for European carriers, though its not a complete and total system shutdown - though once the cloud is gone, so is the problem, which was not true of 9/11. For US carriers that serve europe, it probably puts a dent in the revenue stream, but likely not a gigantic one. For those that don't serve Europe or codeshare there, probably little to no effect.