the correct way to do this is to set up Budgets / API limits. This is quite hidden and requires you to submit a request form. When I tested it, it was granted immediately. I'd guess this happens when requesting lower limits instead of higher than default limitations.
I've had this check in place and it didn't work lol
At least the bill was only 30€ but it changed my perspective.
The issue was that the pubsub topic wasn't triggered. I don't know why, in testing it worked. It might have to do with the way the resource calculated the cost but I don't care any more. I've come to the conclusion that the only way to keep costs in check is to know wtf you are doing when setting up resources. This is a shitty way but as long as google doesn't implement an off-switch for billing this is the best/only way. I hate it but i'll live with it.