We are building a service that utilizes the Gmail API. In order to understand our costs as we scale, I would like to know how much it costs to use the Gmail API. I've followed the instructions at https://developers.google.com/gmail/api/v1/reference/quota through to the point at which it says:
If you have enabled billing for your project [we have], clicking Quota takes you to a page where you can view and change quota-related settings.
The only option on that page for changing our daily quota is to "Apply for higher quota"; however, clicking that opens a window that says:
Please be sure to review the existing quota limits to confirm you need more than the daily default.... If you simply have a question on limits, please ask it on the Stack Overflow forum
Thus, I am asking here: what is the cost per API unit when one's needs exceed the daily free quota?
The API isn't marked as "billable" meaning it's free up to a limit and there's no set/published pricing above that. If you are using your existing quota or are getting close and want to ask for more, I think best place is to ask on the quota request form. It's quite reasonable to ask for quota to provision for a few quarters of growth IMO and if you're migrating from some other API (e.g. IMAP, atom feed, DOM hacking) then obviously that should be quite reasonable to provision all that beforehand as well.
来源：https://stackoverflow.com/questions/26615431/gmail-api-quota-units-cost