If you go to a store and badger them about this, they whip out a ready-made chart showing the two-year cost of the phone over the contract life. With the lower monthly fees you come out ahead with Sprint. Plus, they have lower early-term fees and one-year upgrades for some customers.
All of the third-party retailers are cheaper than the company stores, because they take their comission and apply part of it to the phone price. Online, like Amazon or Newegg, is usually way cheaper than the company stores.
Not defending one way or another, but that's how Sprint works.
--Qfg