You're paying for Apple's integration testing.
For instance, anyone who has tried to put a Western Digital Scorpio drive in a MacBook is finding out that its built-in motion sensor doesn't play nice with Apple's motion sensor, producing kernel panics if the machine is moved enough to trigger SMS.
But I assume you do all of your own integration and regression testing every time you purchase aftermarket hardware, right? Or do you just assume that everything will work and then blame Apple when it doesn't?
The cost of a computer system is much more than the cost of the individual components.
Your last statement only applies to the uneducated consumer, like people that get underbody coatings on new cars, it's a waste of money.
I taught my 4 year old to build a computer, he can identify the parts and what goes where all I have to do is help with the "muscle". If he can do that, I don't see why anyone can't do a simple search on a product to see if it will work with their computer. Apple takes advantage of the consumer more than most companies simply because of who they market to.
I've been repairing macs since before the powerpc was even released. If you spend a few minutes researching something you can usually find the answer.
The only difficult thing on an apple product to fix is the video boards on the original iMac. I've got a couple G4's and powerbooks, about the only parts that are still factory are the motherboards and cases.
It's no harder to find a part to upgrade a mac than it is to find the right number spark plug for your car. And it's easier to upgrade your pc than change plugs
I guess I'm just saying I don't want to pay for "integration testing" to make sure my dvd player will play dvds released after the player.
P.S. Find out what brand product apple is using for the prospective upgrade, buy the same or similar model in an oem version and save enough to still feed the kids. Course I just flashed a Radeon 9700 pro from an old pc to work in one of my G4's, maybe I'm biased. Didn't have to pay $150 more for apples version (at time of release that is.)