Question: I am a software developer and I've been working on integrating Microsoft Graph Api in my application to be able to retrieve calendar information for users. For development purposes, I have been using a Microsoft 365 Developer subscription, in which I register my app, by following <https://learn.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app>. This seems to be working fine with my developer subscription, so after creating a small proof of concept, now I am trying to make a time and cost estimation for this, and I have some doubts about if there are any costs for the client.
Is Azure Active Directory Admin Center available on all Microsoft 365 business plans with the "App registration" option? Or are there any other cost-related aspects I need to take in consideration?
I've searched online and found that there shouldn't be any additional costs, but since I am not really familiar with Microsoft 365 and Azure Active Directory Admin Center other than what I used for my proof of concept, I am not really sure I have the right information.
Maybe there is someone who already implemented this in production and can have a better understanding over this.

Thanks

Answer: 
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> Microsoft 365 subscription extra-costs for Microsoft Graph Api usage
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1.There is no extra cost to use the API, it is open source.

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> and I have some doubts about if there are any costs for the client.
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2.No,it's free.And only microsoft 365 subscription needs to be charged.

3.Yes,all Microsoft 365 subscriptions offer this option.

As @Nishant - MSFT Identity says,Registering apps in Azure AD does not involve any costs and Microsoft 365 E5 developer subscription​ comes with Azure Active Directory(it's free) for building advanced identity and access management solutions.
M365 subscription will have an Azure tenant associated with it and hence you can create application for authenticating to AAD using MS Graph.