I'm sure cost also factors into Apple's decision making, but your statement that OLED is far superior to LCD is based upon what exactly? You have to weigh all factors and obviously Apple's decision has been that there are too many negatives with current OLED technology to switch. And as far as price goes, Apple drives price better than anyone, and if they chose to go OLED, they'll get screens at the price they need.
Frankly, there are crappy OLED screens on the market today, as made obvious by all the give-away Android phones on the market using OLED. Again, they're supposed to be "better", but no company is putting out OLED phones that have 3 day batteries (without including giant batteries in their phones), so where exactly is the "superiority" of OLED?
I would suggest, that the superiority you speak of is in principle only and when weighed against the whole, they're not much different than LCD.