Not sure how you are determining this, or what phone you are looking at. Compare the price of the phone with your 2 year contract extension to the price if you select "month to month". Even for my non primary lines it tends to be a few hundred dollars, although it varies depending on the phone and location I believe. Either way I would be willing to bet it is much more than $30.
It also depends on what plan you have. If you are on a higher plan you get a larger discount. And yes, you get huuuuuuge discount to begin with!