APP Engine for Meta API Conversions - Billing?

[CloudPGE]

Hi!

We are trying to implement Meta API Conversions through the "Partner integration" with "GTM server-side". We carefully followed the instructions and, good news, it seems to work.

The only thing we need help with is the Google Cloud Billing. Indeed, to do the setup of this Meta API Conversions, we had to create a Google Cloud Console account and Google CLoud Project (APP Engine).

For now, it's free, and if I understood well, we get like 200$ of credit also, but at some point, that will be done and we will have to pay.

That's my question: how much we will have to pay (by month / year)? How can I understand that? The document is hard to understand. We litteraly just want to have this Google Cloud Project we created active (so the Meta API Conversions work), and that's it.

Thanks a lot for your help,