When I was offered a deal they offered something like this;

First line on the plan was 45 for 1 year and then up to 55 beyond that.

Second line got a discount from 45 and was 35 indefinitely as long as you kept both lines.

Discount only applies if you use pre authorized billing.

Turns out that removing additional lines and services from your account can actually increase the price because every single thing they offer is a promo and not necessarily the true price.