Hello,
Based on my understanding, the purchase of RDS CALs typically incurs a one-time cost per license, rather than a recurring monthly payment. However, according to the limited information I found in Remote Desktop Services - Choose how you pay | Microsoft Learn, it suggests that for service providers (HSP or MSP) or ISVs, opting for the per-user SALs license offers a flexible, pay-as-you-go model.
We apologize, as tech support, our expertise primarily focuses on addressing technical issues, and we can only provide basic information regarding purchasing CALs. For the most accurate details, we recommend reaching out directly to your Microsoft Licensing representative or consulting the official Microsoft documentation for the latest licensing terms. You may find our licensing documentation helpful at the following link: https://www.microsoft.com/licensing/docs/view/Windows-Server
Regards,
Karlie