https://phys.org/news/2018-11-apple-iphone-trade-in-values-entice.html

The new values, which extend to devices ranging from the iPhone 6 and 6 Plus all the way to last year's iPhone 8, can net you anywhere between $150 for a 6 and $200 for a 6 Plus to $300 for a 7 Plus or an 8, assuming your phone is in good condition. For reference, prior to this boost, Apple was offering just $75 for an iPhone 6 and $100 for a 6 Plus.

The iPhone 8 Plus and X are not receiving boosts in their trade-in values of $350 and $500, respectively.