You require large users to pay more for the same functionality, that $5/mo user gets.
I would create 2 plans: 1) Regular, $9/mo per user 2) Enterprise, $29/mo per user with some added functionality, like custom login page, branding and some user management/sub-users.
This way you will give an actual value to enterprises, that justifies paying more, while keeping smaller users happy, by removing confusing pricing.