I think the point most people are missing is that, sure it is cheaper to buy from an alternate reseller and install yourself, however the average Joe, isn't able to install himself. The other thing is that you are paying for the labor of a professional to install the item, as well as (in this case) the apple care provided with the item. I have a feeling that apple would frown at opening a computer and seeing 3rd party ram.
Another note is that if you take apple.com and dell.com and compare computers, apple's are actually cheaper for what you get. Granted you need to compare EXACT components. Dell likes to cut corners on features most people fail to look for... such as cache on the processor. Go ahead, try it. The problem most people think to be apple being so overpriced is a misconception, really they just don't carry a low end model, they only go down to upper mid-range.