It would cost Europe trillions, but it would also cost the attacker trillions. Russia has shown itself to be utterly outclassed. A conventional war with the EU

It would cost Europe trillions, but it would also cost the attacker trillions. Russia has shown itself to be utterly outclassed. A conventional war with the EU would be barbaric and expensive but short. China has sufficient forces to attack Europe, but zero motivation. It would be a trillion-dollar war if it happened, but it won't. The most likely scenario for Article 5 is the same as last time: a rogue state (probably via a non-state proxy). The resulting war would be asymmetric, but Europe could do it with existing forces. It would cost tens of billions but not trillions.