Logging in with AAD: B2C or not, what license costs?

After reading a bit more about AAD, I want to renew my question as I didn't get any responses anymore in my earlier post.

Our company is working on a webapplication that holds privacy-sensitive data.
Therefor we're looking for an appropriate way to secure the login to it.

Our product aims at about 500 users, which are customers, not users within or related to our company.
We only need to provide them a secure access to the website we're creating for them. That's all, no sharing or interacting with them.

However, our users are known to us. They are employees of a variety of completely different companies, with no link to eachother. For some bigger companies a few employees will get a login, for smaller companies only one user will get a login.

If bigger companies would have their own tenant, the application should be coupled to their tenant. For all small companies without one, we would create one basic aggregated tenant.

My questions are related to the license fees:
- Would this be B2C functionality?
If so, am I right that the costs would only be €0.026 per authentication (because we want MFA enabled)?
This is based on this page: https://azure.microsoft.com/en-us/pricing/details/active-directory-b2c/
Or do these costs come on top of another Azure license that I'm not aware of?

- If not, I reckon the costs would come from this page: https://azure.microsoft.com/nl-nl/pricing/details/active-directory/
And then the costs for 500 users including MFA would be ~ €37000, as we would need a Premium P1 license to enable MFA.
That's a totally different level than the costs of the B2C functionality...

- If the latter is the case, and we would have some customers that allready have their own tenant. Would we be able to connect them to the application for free?!
Is that how it works? So if you're creating a webapplication that allready has a tenant, you would be able to cuse AAD for free, and if they don't have a tenant, it would cost like €60 per user per month?
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Cliff GaliherCommented:
You really need to. Get your developers and a Microsoft rep into a room (conference call with Ms) and iron out your strategy. From what I'm reading, you don't want native AAD or B2C. *Maybe* B2B, but even that would be app specific. Getting pricing for a product that won't fit your needs is a bit of a cart before the horse situation.

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