It is plain simple (and sad). Apple, as other Apple-related manufacturers like LaCie (for storage devices) charge 2x to 3x the market price (eg., Amazon) for exactly the very same make of model of components that they do not manufacture like RAM, disks, etc.
The only workaround is to buy from other manufacture (not possible if you want a Mac) or to buy the cheapest model and then make the reseller upgrade for you or DIY upgrade, if possible (not soldered components, etc). That way you can save huge amounts of money.
In relation to that, I wish I could purchase LaCie SSD enclosures and then place the corresponding Samsung SSD that they use inside, resulting in the very same product at half or less the price.