You're actually charging more per user for more users ($2 per user for 5 users; $2.50 per user for 50 users), rather than giving larger accounts a discount. I imagine the reason for it is the load it places on your infrastructure, but I've rarely seen that kind of price stacking work in the wild.
Past the first plan, the pricing is all $2.50 per user. I like to think of the first plan as the "startup" plan so we made it cheaper. We do offer discounts per user past 50 users.
The pricing strikes me as a bit odd though.
You're actually charging more per user for more users ($2 per user for 5 users; $2.50 per user for 50 users), rather than giving larger accounts a discount. I imagine the reason for it is the load it places on your infrastructure, but I've rarely seen that kind of price stacking work in the wild.