My misunderstanding is only because AT&T refuses to make the total costs clear for a given plan. It's just bits of monthly charges, which will inevitably be added to other charges and fees. That's because they want to hide the true and final cost from their customers.
This means nothing. It doesn't tell me nearly what _my_ bill will be.
It's pretty easy to figure it out if you just go on their website and add what you want to the cart.
For example, I just went on and selected Mobile Share 2GB for $40 per month. This is the plan/data cost.
It's $40 per line to add a line, but if you do next, you save $15 per line. This is the line cost.
With the AT&T Next, a 16GB 5S is $22.92 per month. This is the phone cost.
$40 (data) + $25 (line) + $23 (phone) = $88 plus tax.
It gets cheaper the more lines/devices you add.