How big of a difference will vary from model to model, but basically save for the iPhone X, all models have had their trade-in values lowered. Take for example the iPhone 12 Pro Max. It was initially valued up to $790, but it is now only worth up to $700. This is a $90 drop, which is somewhat substantial, but like we said, it varies from phone to phone.
For example, the second-gen iPhone SE was worth up to $170 but it is now only worth up to $160, so that’s only a $10 difference, but then again, we suppose phones that weren’t worth much to begin with are seeing small changes in value compared to Apple’s newer and more expensive models.
Keep in mind that these are the max prices you can get for the phones. How much you will get will also depend on other factors like the condition of your phone, so you could actually end up getting less than the max value if you were to trade-in a beat up old iPhone versus one that’s still in relatively good condition.