Apple charges way more than market rate for storage and memory upgrades. A stick of memory that costs $40 on Newegg might cost $200 from Apple.

In the past the sensible option was to buy a Macbook with stock memory / storage and upgrade it yourself, but of course that's no longer possible with the rMBP. Still, that says no more about the actual cost of SSDs than the price of a hotel room says about beds. :)