Regarding the cost of the instance of the tenant subscription 

Term Definition
Tenant For model-driven apps in Dynamics 365, a tenant is the account you create in the Microsoft Online Services environment when you sign up for a subscription. A tenant contains uniquely identified domains, users, security groups, and subscriptions and can contain multiple instances. Cost please 

Instance When you sign up for a trial or purchase a subscription, a Production instance is created. Each additional Production or non-Production (Sandbox) instance you add creates a separate and isolated organization on the same tenant.Cost please 

Multiregional instance An instance in a different region than where your tenant resides. Local instances can provide quicker data access for users in that region. Cost please 
Subscription A subscription consists of the licenses and add-ons included with the trial or paid service you signed up for in your account. Subscriptions can vary in license type, price, and end date.

For example, a subscription might be 100 licenses of Dynamics 365 Professional and 10 licenses of Dynamics 365 Enterprise.Cost please 
Identity The user account used to sign in to model-driven apps in Dynamics 365. You can also use this identity to access other Microsoft Online services, such as Office 365 or SharePoint Online. Administrators can decide if they want to federate user identity management between model-driven apps in Dynamics 365 and on-premises Active Directory.Cost please 
User account A user account assigned by an organization (work, school, non-profit) to one of their constituents (an employee, student, customer) that provides sign-in access to one or more of the organization’s Microsoft cloud service subscriptions, such as Exchange Online or model-driven apps in Dynamics 365. Access to an online service is controlled by the license assigned to the user account.

User accounts are stored in an organization’s cloud directory within Azure Active Directory, and are typically deleted when the user leaves the organization. Organizational accounts differ from Microsoft accounts in that they are created and managed by admins in the organization, not by the user.Cost please 
Security group If your company has multiple instances, you can use instance security groups to control which licensed users can access a particular instance. Cost please