If you didn't get what trotter was saying then, Ill lay it out for you, IMO you need at the LEAST 500$ to build a computer, complete with moniter, keyboard, mouse and speakers, and thats not even Close to a gamming computer.
For a gamming system I would save up at LEAST 850$.
And that would be a low end Gamming system at that.
(At least for all the new games comming out.)
Building a 200$ computer is "Possible" but is it worth it?
Probably not, Unless, the only thing you want it to ever do is word process and email...