The price depends on whether the services you wish to use require the free tier, the Premium P1 license, or the Premium P2 license. As you correctly mentioned, the billing for B2C itself is based on monthly active users. You will also accrue charges for resources that you create in any of the tenants.
For reference, see Azure AD B2C Billing.
You can refer to the Azure External Identities pricing calculator to plug in your numbers and get an estimate for your scenario. https://azure.microsoft.com/en-us/pricing/calculator/
For the subscription, you would likely want to use a Pay-As-You-Go unless you plan on deploying a lot of Virtual Machines. For subscription purchase options, see the Purchase Options and Pay As You Go pricing pages.
Microsoft Q&A does not directly support pricing questions. To get an estimate for your specific needs, I would highly recommend speaking with a representative from the billing team. To reach out to billing support, see: https://azure.microsoft.com/en-us/support/options/
In addition, here is the phone number for the sales team: 1-855-270-0615