The memory modules of the Mac Pro are *NOT* standard ECC DDR. They belong to the faster and more expensiveFB-DIMM family.
http://en.wikipedia.org/wiki/Fully_Buffered_DIMM.
By the way I don't the OP said that Apple upgrades had reasonable prices, but the systems themselves, which is partly true. Especially if you account for all the things that they come with, applications, ports and design/robustness. Just check Sony Vaio prices up :P

Peter

Apple will either release a cheap macbook ($799) with discrete nvidia graphics, aluminum casing made with water-jets and lasers, netbook-sized, featuring an LED backlit screen -with good panel- OR thousands of people on the internet will start writing about how disappointed they are.