Hello Amir Shiloh,
Thanks for your question.
Yes foundational CSPM is free. If you're having costs, I'd recommend you go to Azure cost management to view the exact details of what you're incurring on the Defender for cloud.
If you have charges it could be as a result if:
Any additional services (e.g., Defender plans for VMs, SQL databases, etc.)
Ensure your Defender-related logs are not exceeding your workspace retention quota or ingesting excessive data
Review the Defender for Cloud pricing page to learn about Defender CSPM pricing.
If you review the above and still can't trace why, you could open a billing ticket for free here:
https://azure.microsoft.com/en-us/support/create-ticket
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola