Project X

Google’s Text-to-Speech API is priced based on the number of characters sent to the API to be synthesized into audio each month, also depending on the type of the voice used.

How to implement this pricing model for your API without writing any code?
Check out this article.

https://medium.com/@projectxbiz/google-like-text-to-speech-api-pricing-with-project-x-9c66a66d31ee

For APIs which imply a significant incoming traffic from customers (e.g. image processing API, file hosting API, etc.), the ability to bill customers for the amount of uploaded data is an absolute must.

https://medium.com/@projectxbiz/api-billing-for-uploaded-data-volume-ec4571bfe30