IHS estimates that a notch-design increases the production cost of a smartphone display (whether LCD or OLED) by more than 20%. A notch-type 5.9" OLED (like in the iPhone X) costs $29, 25% higher than the cost of a regular 5.8" OLED.

Full display vs notch-type cost comparison (May 2018, IHS)

IHS explains that cutting the notch results in yield loss, which is the main driver of the higher costs (the panel itself, of course, is also larger before cutting the notch in the comparison IHS makes). In theory, cutting a plastic OLED is easier than the glass-based LCDs, and IHS forecasts that manufacturing costs for notch OLEDs will fall far more rapidly than the costs of notch LCDs.