Hi @Anonymous
Welcome to the Power Pages / Power Portals forum.
I will try to explain the licensing in a simple way. There is a slight misunderstanding in your calculation.
1. Assigning an internal user's license to External users is not possible. You can add tenant B as the guest user in your tenant. But Assigning licenses is not allowed for guest users. Hence, you need to buy "Login capacity Authenticated users"
2. Adding the official screenshot below, for your 25 users, you need to buy Tier 2 which cost 100$ per unit.
In simple terms, 10*$100 = $1000 per month

Hope it helps.
------------
If you like this post, give it a Thumbs up. Where it solved your request, Mark it as a Solution to enable other users to find it.