... and most people would naturally use the cheapest SD cards they find, causing slow app startup times and stuttering HD video playback. And what would happen when the card fails? Besides, managing what to store in the card and what in the internal memory would be a hassle. There are many good reasons why there are no memory card slots.
Though I agree the price of storage is high, but it's more related to Apple's business model.
If they were to do that I think it would be somewhat integrated so that the SD card's additional space isn't used until the phone's built-in storage is all used up. Sure, some cheap cards could go bad but it's not like those things have super high failure rates or anything. When it comes down to it, Apple doesn't offer options like this because they make a killing off their $500+ tablets with a few USB sticks worth of storage space.