Do prices of older iPhones decrease after the release of a new model?
During the iPhone 11 announcement, Apple said they were reducing the price of the iPhone 8 to starting at $449 and the iPhone XR to starting at $599. These price changes are currently effective on Apple's website.

Historically, Apple announces new models and pricing and at the same time, refreshes their official pricing the same day as the keynote / press release. Additionally, look online and at Apple stores as they then sell (often at further discount) any now “discontinued” models in the Certified Refurbished / Clearance section: 


https://www.apple.com/shop/refurbished