For $200 or so, you aren't going to be able to buy a subwoofer. You are going to get a woofer in a box, with an amp, period.
With your setup, a PSW10 or 12 (if you can find them at your pricepoint) will fill out the low end quite nicely. All Polk subs are rated (measured) in an anehoic chamber - in room response is usually better.
That being said, I've heard GREAT things about the Dayton offerings, and I've been a PE regular for going 7 years. I've also ran a pair of X-Sub's for a music setup (Danny Richie is a good friend of mine), and they certainly rank in the 'bang for buck category'.
Bottom line, you get what you pay for, and within the budget, you need to look for closeout deals, and sale specials - Polk, Dayton, X-Series, etc etc. I can't even recommend a DIY route without at least doubling the budget.
Cheers,
Russ
Polk Audio CS