Earlier quoted context omitted.
Seconding a sibling question: What $50k API fee? To access your gmail? I've been using gmail since 2008 or so without ever touching their web/app interface or getting an API key. You just use it as an IMAP server.
To use Google's sensitive APIs in production you have to certify your product and that costs tens of thousands. To be honest, didn't think about imap at first, but it looks like that could be getting tougher soon too https://support.google.com/a/answer/14114704?hl=en . Soon they will require oAuth for imap and with oAuth you'll need the certification: https://developers.google.com/gmail/imap/xoauth2-protocol . If it'…
> You will no longer use a password for access (with the exception of app passwords)
I'm not seeing anywhere that I'd need to pay money to use OAuth via an app like Thunderbird or another email client. That app would either need to support using OAuth to let the user auth and get credentials, or use an app password.