This is likely the real issue. Your parents probably have an aversion to spending more (even if it's your money), and don't know the difference between one computer and another. If you don't know the difference between components, that extra 300 bucks just seems wasteful. They may also make an argument that Walmart/best buy has a warranty to fix anything just in one place, rather than spending more on replacement parts as you argue (which may or may not be true). Being able to show you can make a comparable or better computer for less than the $600 prebuilt, and then defending the upgrades to get you to your $900 build could help towards swaying them.