For the basic plan, it's [$20+$10/GB]/user/month for 1 user; [$17+$10/GB]/user/month for 3 users; [$16+$10/GB]/user/month for 6 users. So that's a max of a 15% discount for 3 or 20% discount for 6 (when not using data; less of a discount with data-usage).
Then for the next plan, it's $60/user/month for 1 user, $30/user/month for 3 users, and $30/user/month for 6 users. That's a 50% discount from 1 to 3, then no further discount for moving to 6.
I'd guess that the basic-plan is meant for price-sensitive subscribers, so they tried to keep it as cheap as possible, making further discounts difficult. Presumably they can still offer a discount for bundling users since it'd presumably save them by having fewer accounts to manage.
And then I'd guess that $60/month/user to $30/month/user might reflect a premium in the single-user plan. But presumably the sharp drop to a 50% discount by 3 users isn't due to reduced account-management costs (as it's not reflected in the basic-plan's discounts), so.. more competition in that market? Or maybe middle-tier subscribers are more likely to be price-sensitive if they're in a situation where they'd have multiple users on a plan? But then, why no discount for 3 users to 6?
In short, I wonder how they arrived at those price-points? Or, if those are affected by market-competition, I wonder why the market would've pushed them toward those price-points?