Assuming it works the same as all the other API’s then it will be pro rata, i.e. 10 chars is 100th of 1000 so would be 100th of $0.015 or $0.030 so 0.00015 or 0.00030, when those amounts add up to 1 cent you will see it showing on your balance.
You could try a test that ran a 100 character speech generation and then check your account, it can take a few mins to update. Then try doing that generation 10 times over and see if you get a 1 cent bill, do it again and if you get a 3 cent bill then you know that’s how it works in reality.
OpenAI bills by very small units. Individual $0.001 calls to embeddings models are accumulated by token counts.
Here’s a post of mine analyzing whisper, the other direction, also reaching that conclusion. Two days before the usage page was destroyed to prevent auditing.
The only thing that is still ambiguous about granularity is the per GB price of assistants’ retrieval documents.