I'm not sure the carriers have a say in what Apple charges. It's probably the market. People go nuts for the iphone. Lines still happen for the iPhone. That's what sets the price. I don't know how many lines were made for the Nexus. As awesome as Nexus or any other phone may be, they're definitely not on the same market level as the iPhone, in terms of popularity not in terms of percentages.
A market segment that makes up a smaller and smaller proportion of the market goes nuts for the iPhone.
That means that carriers have less and less incentive to care about whether or not Apple walks away from the negotiating table if they ask for lower prices.
Apple will still have the diehard fans who are willing to buy unsubsidised directly, if the iPhone market share dips low enough that carriers see the iPhone as optional enough that they feel they can afford to lose it, Apple will have to choose reductions in sales or lower prices.
