The cheapest plan for adding your computers to Azure Active Directory (Azure AD) is the Azure AD Basic plan. This plan includes the following features:
- User and group management: Create, manage, and delete users and groups, and manage group membership.
- Authentication and access control: Control access to cloud and on-premises resources, and authenticate users with multi-factor authentication (MFA).
- Device management: Join devices to Azure AD and manage them with conditional access policies.
- Application management: Control access to cloud and on-premises applications, and enable single sign-on (SSO) for users.
- Self-service password reset: Allow users to reset their own passwords without the need for IT assistance.
Azure AD Basic costs $1 per user per month, and allows up to 50,000 objects. It provides the basic features of Azure AD and it's suitable for small to medium size businesses.
It's important to note that if you want to use the device management features to join your computers to Azure AD, you'll need to have a Windows 10 Pro, Enterprise or Education edition device, and you need to run the Azure AD Join process on each device.
Also, you may consider looking into the free version of Azure AD which is called Azure AD Free that provides some basic features like user and group management, Authentication and access control, device management.
It's always recommended to check the different pricing plans and features that Azure AD offers to ensure that you are selecting the best one that fits your organization's needs.