Apple has been pricing on what the market will bear for a long time, maybe the entirety of the iphone’s existence. Prices may go up; they may not.
Apple will not be financially obligated to increase prices as a result of cost changes: an iphone costs something in the $300-500 range to manufacture, and Apple charges $800+ — even a doubling of the cost of the SoC will not fundamentally alter Apple’s pricing calculations.
Price increases for the 15 will be determined entirely by if Apple thinks the market will bear that price increase such that doing so would result in more profit for them.