Text-to-Speech is priced monthly based on the amount of characters to synthesize into audio sent to the service.
Note that Speech Synthesis Markup Language (SSML) tags are included in the character count for billing purposes. For example, this input counts as 79 characters, including the SSML tags, newlines, and spaces:
<speak> <say-as interpret-as="cardinal">12345</say-as> and one more </speak>
|Feature||Monthly free tier||Paid usage|
|Standard (non-WaveNet) voices||0 to 4 million characters||$4.00 USD / 1 million characters|
|WaveNet voices||0 to 1 million characters||$16.00 USD / 1 million characters|
Google Cloud Platform costs
If you use other Google Cloud Platform resources in tandem with the Text-to-Speech, such as Google App Engine instances, then you will also be billed for the use of those services. See the Google Cloud Platform Pricing Calculator to determine other costs based on current rates.