For a limited time Apple is offering up to US$100 of additional trade in credit when you trade in certain iPhones. This can be used towards an iPhone XS or iPhone XR.
Trade In Credit
Models available for trade in credit include: iPhone 6/6+, iPhone 6s/6s+, iPhone 7/7+, iPhone 8 (but apparently not iPhone 8 Plus). In the regular trade in process Apple offers US$150. For more information visit the Apple GiveBack page.