Hello @Abdelrahman Mokhtar , Thanks for using Microsoft Q&A Platform.
If you have created a "Speech service" with "Standard paid (S0)", then you will be billed based on the number of characters you use. Here are the pricing details: https://azure.microsoft.com/en-us/pricing/details/cognitive-services/speech-services/
If you are using a free (F0) speech resource. The upper limit of your free quota is 500K characters every month for neural voices, with each file limited to 3000 characters. This is not included with the standard pricing. You can start with free tier and can switch to standard tier whenever you needed.
Here is the Azure pricing calculator you can estimate the price before you use it: https://azure.microsoft.com/en-us/pricing/calculator/
I hope this helps.
Regards,
Vasavi
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