Apple always goes on margin. The extra memory versions of the iPhone are all margin.
When you get to the $850 iPhone with 64GB of memory, the profit margin is generally around 75%.
As in, it costs them $215 to make a $850 device.
Apple rarely dips below 40% margin on it's products it seems.
Now that Apple has over $100,000,000,000.00 in it's little hedge fund and has become the largest fund in the world (iirc), I've begun to look at the iPhone as the worlds most successful hedge fund capital raising scheme.
"Always" might be too extreme given the iPad Mini. Also, I would consider an upsell of the same model a different thing from the base price of different models.
If Apple can get China Telecom signed, do you think they'd offer the 5C at a lower price because of the volume? There's 700 million subscribers in that carrier.