This design is meant to be set on a desk and is fully shielded, and can be built for under $100/pr. No amp though. Maybe you could pick up an old stereo receiver for $50 at a pawn shop.
http://www.knology.net/~wesnor/blue.html
2.1 for $200 is a hard price point to hit. Generally, amplifiers alone will take up more than 1/2 the budget, unless you have enough know-how to turn the amp modules at
www.apexjr.com into functioning amps. And if shipping is part of the $200 budget, the fact that you'll probably be buying from different sources drives shipping above $20.
You might end up spending $250-$300, but you will have a very impressive computer speaker system.