Overpriced SSD components?
Let's just face it -- Apple's upgrade pricing on the Retina models is ridiculous. Around $500 difference between 256 and 512GB SSD? That's RETAIL for an entire Samsung 512GB in its own 7mm casing. It's arguably even worse for the upgrade to 16GB of memory -- the price is equivalent to the entire 16GB, not the difference, and not wholesale but full retail, and not street, but list price.
If there is a smaller difference between base and upgrade on the 13 inch model, let's hope it's because Apple is finally coming around on these ridiculous markups. As things stand at present, better just to get the base model and use external storage.