In the summer when the new iPhone comes out there will be a glut of second hand iPhones on the market, and you will be able to get a sim only contract. My iPhone is now paid for and my contract is £20 a month which lets me have unlimited data (internet).
Likewise, the current iPhone carriers are not really competing on price at the moment, but when they have the new phone, they will do, and there will be some movement on price.
The rest of the smartphones have still not produced a phone as versatile and as easy to use as the first iPhone released 3 years ago, so while those rival products are inferior, Apple can charge more. Until the competition makes something that genuinely competes with the iPhone, Apple can still charge a premium.