For information, I received an official reply to a support question to Google Cloud Support:
Q. First, I understand that there is a quota usage limit per month. Is this limit per
project or per billing account (i.e. all the projects in the account)?
A. It is per billing account, regardless of number of API keys or projects in it.
Q.Secondly, how does the $200 'free' usage work?
This answer was a bit more complicated, but essentially the Google Maps $200 'free' usage is a notional amount which equates to a number of free loads (which varies depending on what is being used). In our case, for example, we would have up to 28,000 loads of Dynamic Maps per month. Then there is a rate for up to 100,000 (in our case, $7 a thousand) and a rate for over 100,000 (in our case, $5.60 per thousand).
So, to take your example of 129,000 loads a month, assuming they are all Dynamic Maps.
28,000 would be free
100,000 would be charged at $7 per 1,000 = $700
1,000 would be charged at $5.60 per 1,000 = $5.60
Your total cost in that case would be $705.60 per month.
The link for maps pricing is here:
[
cloud.google.com...]
He also discussed something called 'gap credits' where they had measured an account's usage for a 24-hour period and would use that usage as a standard for the next two months after June to allow businesses to transition to the new structure.
In other words, if you had 3,000 loads in that period, you would have ca. 90,000 loads which would not be charged for the two months following the introduction of the new pricing.
One of the friendliest and most informative support answers I've had from anywhere, to be fair to Google.