You'll have to either call them up, check online (they'll list the regular price of the phone, and how much you'll get taken off with a contract and any other discounts that my apply but this only for online though), or go to a corporate store. Please don't go to the other stores they are a bit shady to deal with, there always trying to get me to buy their phones and pressuring me to everytime I seriously walk by their kiosk thingy.
But usually you do save a good bit of money like $50-$100 maybe even more depending on the phone and how long you want to extend the contract for.