Cost of Google Cloud GPUs


in order to be able to use a GPUs on Google Could I upgraded from free to paid cloud services and increased my GPU quota.
However when trying to attach the GPU to my notebook it tells:
“Estimated cost difference US$321.24 monthly”

Does that mean they will charge me US$321 per month form the moment I attach the GPU to my notebook?
If yes, how can I change this to a usage based model?

Thanks for your help.
I tried top contact Google Cloud support, but I always land at the billing help bot or I am asked to upgrade to paid support. That all seems to be very costly for a hobby user.