You're smoking crack, mass produced PC's aren't cheaper because mass produced PC's still source their materials from different manufactures such as ram and hard drives etc. Mass produced PC's have prices that allow the manufacturer to make massive profit margins. Buying your own parts means you're getting wholesale or close to wholesale prices which means you can have the same exact set up from a mass manufacturer for a lot less.
I'm sorry to say but your stupid for thinking pre-built is cheaper. The brand names don't even have good build quality. I can make my own computer and it will be more reliable, faster and outlast any pre-built computer.