Hi @Kavin R ,
Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.
Registering apps in Azure AD does not involve any costs and even if any of its related services are hosted elsewhere other than azure. And if your monitoring app making any API calls, you don't have to calculate cost per API call made to the application.
If you are using Microsoft 365 E5 developer subscription​, it comes with Azure Active Directory (it's free) for building advanced identity and access management solutions.
Hope this helps!
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