Note that this is an Australian perspective.
For a bog standard mini tower PC putting it together is not much more difficult than buying and assembling Lego.
If you want specialist gaming or "whatevering" PCs then you do need to put a lot of thought into it.
IN OZ the unfortunate thing is that you build for pleasure not profit. You can buy the same core components in a system, pre built, and cheaper, at your local computer faire.
Bummer.
My main point is that most of the above comments are adding too much complexity to a simple problem.
I have been building tower pc's for years. I have knowledge.