Amazon has the Crucial M4 512gb and the OCZ Agility 4 512 gb for the same price. They are trying to one up each other and we are the beneficiaries.
Retina-based MacBook Pro has two types:
15-inch 2.3GHz Ivy Bridge with a 256GB SSD (US$2,199)
15-inch 2.6GHz Ivy Bridge with a 512GB SSD (US$2,799)
A 300MHz faster processor sometimes might justify US$600 higher cost, but most would be willingly pay $100 or $200 max. Perhaps Apple decided on the price of those laptops long when NAND cost was much higher. I need to save to get one anyway.