The SSDs in the Air are replaceable, but new units are insanely expensive. I've been looking to swap out my 128GB SSD with a 256GB unit (Mid-2013 MBA). An OEM 256GB SSD is around $450, the OWC Aura (seemingly the only compatible third party unit) is $250 -- better, but still hard to justify. There are used units on eBay, but I don't like using second hand hard drives.
Are there any other options out there?