Well there's a few things....
You can't really build a laptop from parts, that's only something you would want to do with a desktop.
Secondly, for 400-500 $, you wouldn't be able to get a laptop that can run Minecraft at 80 or higher fps, unless you ran the lowest settings,
Laptop's are more expensive for how good they are to desktops.
Laptop's are more compact, and cost more. You can expect a Desktop that has good parts for around 500$ would cost you 650-750 with the same specs as a laptop. That being said, you might be able to get the fps your looking for, if you know what your doing when building your computer for 500$. My advise for you is to do a little research. You might be able to get what you are looking for 500$, but only a desktop could get you there.
