DIY or semi-DIY(not actually making anything - just combining pre-finished parts together) is always a better value by far for performance vs. $$ in subwoofers. If you are willing to do minor work (cut a hole or two with a jig saw and screw/attach the drivers, etc.), I can specify all of the needed parts/sources/prices to get you the optimum performance for your $$.
Also, remember, best SQ is always had by using twin subs(one on each side of room) and by using an external active crossover device. The Marantz stereo receiver suggested above has the needed connections to integrate the external active crossover. A very good quality crossover can be had for $90USD if you shop wisely.
However, if you are looking to only spend like $200, you can't really do well DIY, that's just too cheap, and you'll end up with something comparable to some of the commercial options around that price anyways. The threshold breaks, really, around the $300-$400 range. You can do it for less, given specific applications/circumstances.
-Chris