iPhone should have a premium price than a Htc One or Galaxy S5 so iPhone 6 need a 100$ add
What kind of none sense is this, are you actually willing to pay extra just to say your phone is more premium or something, if so you really need to re-evulate your life.
Sticking too the OP I currently buy all my phones sim free, I live in London and the market is very different but I would do the same in the US. You get too be much more flexible e.g. I can just change phones whenever and we have really nice 12 months sim only plans so I can change networks if 1 starts to under perform. It is always cheaper, then money saved I can put forward to buying a new phone as and when I please. Much more freedom.