The cost is dependent on what the issue is. And there are some conditions where they won't do a swap at the "repair cost" because of the condition of the device, but might do an actual repair of the device. Like if there's severe cosmetic wear to the case that doesn't prevent replacing the display, but will mean it's not as valuable as an exchange.
Apple may do complete swaps for anything starting with just battery replacements. Back when they were charging $29 for battery replacements, there were reports that some of them were just a complete swap in lieu of installing a battery in an iPhone and returning it to the customer. But they won't do it for anything lower than their "other damage" price if there's anything like case damage.
